Concurrent collection:
ConcurrentMap<LocalDate, A> ex = new ConcurrentHashMap<>();
class A {
AtomicLong C;
AtomicLong D
}
How can I synchronize by locking "C" and "D"? That is, I need to change "C" and "B" at the same time with a guarantee that while I change the other one, the first one does not change from external actions.
Thank you.
What you're solving for
You are describing that you want to:
allow a caller to modify something on an object
prevent any other callers (other threads) from modifying things at the same time
Solution description
This solution uses synchronized, though there are number of other mechanisms available in Java that would support this (several of which are covered in
the Lock Objects section of the Java Tutorials).
The way "synchronized" works is that you designate some code using the "synchronized" keyword, along with an object to synchronize on.
When your code runs, the JVM will guarantee that, for all code which is synchronized – on the same object – only one thread
can proceed at a time.
You can make a synchronized code block, like below. Note: this defines an Object named "lock", but it's just a name chosen for clarity when
reading the code – you could name it anything you like.
Object lock;
synchronized (lock) {
... // all things here run only when "lock" is available
}
You can also designate an entire method as being synchronized, like this:
public void synchronized print() {
System.out.println("hello");
}
This second example behaves like the first - it also locks on an object – but it's not clear at a glance what the object is; that is, how does
the JVM know which object to sychronize on? This approach works if the method itself is called on an object instance, and in that case the lock becomes this. I'll show an example below.
There's good info in the Java Tutorials about synchronized methods.
Solution #1: synchronized block using Object lock
Here are a few notes about a class AllowOneEditAtATime:
it has two private members: one and two
because they're private, they cannot be changed directly – so it would not be allowed to do something like this:
AllowOneEditAtATime a = new AllowOneEditAtATime();
a.one = new AtomicLong(1); // cannot change "one" directly because it is private
defines private Object lock – this is meant to act as the thing that two different synchronized code blocks will lock on. It's totally fine to have different blocks of code each synchronize on the same object. This is the main technique you're after.
uses a synchronized block inside setOne(), synchronizing on "lock"
uses another synchronized block inside the other method – setTwo() – also synchronizing on "lock"
because both setOne() and setTwo() are synchronized on the same object, one of them will be allowed to run at a time
class AllowOneEditAtATime1 {
private Object lock;
private AtomicLong one;
private AtomicLong two;
public void setOne(AtomicLong newOne) {
synchronized (lock) {
one = newOne;
}
}
public void setTwo(AtomicLong newTwo) {
synchronized (lock) {
two = newTwo;
}
}
}
Solution #2: synchronized block, using this
Solution #1 works fine, but it isn't necessary (in this case) to create an entire object just for locking. Instead, you can rely on the fact that
this code runs only after someone called new AllowOneEditAtATime(), which means there's always an object instance, which means inside the code
you can use this. The this keyword refers to the object instance itself, the actual instance of AllowOneEditAtATime.
So here's a variation using this (no more Object lock):
class AllowOneEditAtATime2 {
private AtomicLong one;
private AtomicLong two;
public void setOne(AtomicLong newOne) {
synchronized (this) {
one = newOne;
}
}
public void setTwo(AtomicLong newTwo) {
synchronized (this) {
two = newTwo;
}
}
}
Solution #3: synchronized methods, implicit lock
Solution #2 works fine, but since we're using this as the lock, and since the code paths fit with doing this, we can use
synchronized methods
instead of synchronized code blocks.
That means we can replace this:
public void setTwo(AtomicLong newTwo) {
synchronized (this) {
two = newTwo;
}
}
with this:
public synchronized void setOne(AtomicLong newOne) {
one = newOne;
}
Under the covers, the entire setOne() method is synchronized on this automatically, so it isn't necessary to
include synchronized (this) { .. }
at all. In Solution #2, both methods were doing that, so both can be replaced.
By synchronizing both methods, they will both be synchronized on the object instance (this), which is similar to Solution #2, but with less code.
class AllowOneEditAtATime3 {
private AtomicLong one;
private AtomicLong two;
public synchronized void setOne(AtomicLong newOne) {
one = newOne;
}
public synchronized void setTwo(AtomicLong newTwo) {
two = newTwo;
}
}
Any of the above would work, as would other synchronization mechanisms. As with all things, there are multiple ways you could solve the problem.
For additional reading,
the Concurrency lesson (in Java Tutorials) has good info
and might be worth your time.
I have three different threads which creates three different objects to read/manipulate some data which is common for all the threads. Now, I need to ensure that we are giving an access only to one thread at a time.
The example goes something like this.
public interface CommonData {
public void addData(); // adds data to the cache
public void getDataAccessKey(); // Key that will be common across different threads for each data type
}
/*
* Singleton class
*/
public class CommonDataCache() {
private final Map dataMap = new HashMap(); // this takes keys and values as custom objects
}
The implementation class of the interface would look like this
class CommonDataImpl implements CommonData {
private String key;
public CommonDataImpl1(String key) {
this.key = key;
}
public void addData() {
// access the singleton cache class and add
}
public void getDataAccessKey() {
return key;
}
}
Each thread will be invoked as follows:
CommonData data = new CommonDataImpl("Key1");
new Thread(() -> data.addData()).start();
CommonData data1 = new CommonDataImpl("Key1");
new Thread(() -> data1.addData()).start();
CommonData data2 = new CommonDataImpl("Key1");
new Thread(() -> data2.addData()).start();
Now, I need to synchronize those threads if and only if the keys of the data object (passed on to the thread) is the same.
My thought process so far:
I tried to have a class that provides the lock on the fly for a given key which looks something like this.
/*
* Singleton class
*/
public class DataAccessKeyToLockProvider {
private volatile Map<String, ReentrantLock> accessKeyToLockHolder = new ConcurrentHashMap<>();
private DataAccessKeyToLockProvider() {
}
public ReentrantLock getLock(String key) {
return accessKeyToLockHolder.putIfAbsent(key, new ReentrantLock());
}
public void removeLock(BSSKey key) {
ReentrantLock removedLock = accessKeyToLockHolder.remove(key);
}
}
So each thread would call this class and get the lock and use it and remove it once the processing is done. But this can so result in a case where the second thread could get the lock object that was inserted by the first thread and waiting for the first thread to release the lock. Once the first thread removes the lock, now the third thread would get a different lock altogether, so the 2nd thread and the 3rd thread are not in sync anymore.
Something like this:
new Thread(() -> {
ReentrantLock lock = DataAccessKeyToLockProvider.get(data.getDataAccessKey());
lock.lock();
data.addData();
lock.unlock();
DataAccessKeyToLockProvider.remove(data.getDataAccessKey());
).start();
Please let me know if you need any additional details to help me resolve my problem
P.S: Removing the key from the lock provider is kind of mandatory as i will be dealing with some millions of keys (not necessarily strings), so I don't want the lock provider to eat up my memory
Inspired the solution provided #rzwitserloot, I have tried to put some generic code that waits for the other thread to complete its processing before giving the access to the next thread.
public class GenericKeyToLockProvider<K> {
private volatile Map<K, ReentrantLock> keyToLockHolder = new ConcurrentHashMap<>();
public synchronized ReentrantLock getLock(K key) {
ReentrantLock existingLock = keyToLockHolder.get(key);
try {
if (existingLock != null && existingLock.isLocked()) {
existingLock.lock(); // Waits for the thread that acquired the lock previously to release it
}
return keyToLockHolder.put(key, new ReentrantLock()); // Override with the new lock
} finally {
if (existingLock != null) {
existingLock.unlock();
}
}
}
}
But looks like the entry made by the last thread wouldn't be removed. Anyway to solve this?
First, a clarification: You either use ReentrantLock, OR you use synchronized. You don't synchronized on a ReentrantLock instance (you synchronize on any object you want) – or, if you want to go the lock route, you can call the lock lock method on your lock object, using a try/finally guard to always ensure you call unlock later (and don't use synchronized at all).
synchronized is low-level API. Lock, and all the other classes in the java.util.concurrent package are higher level and offer far more abstractions. It's generally a good idea to just peruse the javadoc of all the classes in the j.u.c package from time to time, very useful stuff in there.
The key issue is to remove all references to a lock object (thus ensuring it can be garbage collected), but not until you are certain there are zero active threads locking on it. Your current approach does not know how many classes are waiting. That needs to be fixed. Once you return an instance of a Lock object, it's 'out of your hands' and it is not possible to track if the caller is ever going to call lock on it. Thus, you can't do that. Instead, call lock as part of the job; the getLock method should actually do the locking as part of the operation. That way, YOU get to control the process flow. However, let's first take a step back:
You say you'll have millions of keys. Okay; but it is somewhat unlikely you'll have millions of threads. After all, a thread requires a stack, and even using the -Xss parameter to reduce the stack size to the minimum of 128k or so, a million threads implies you're using up 128GB of RAM just for stacks; seems unlikely.
So, whilst you might have millions of keys, the number of 'locked' keys is MUCH smaller. Let's focus on those.
You could make a ConcurrentHashMap which maps your string keys to lock objects. Then:
To acquire a lock:
Create a new lock object (literally: Object o = new Object(); - we are going to be using synchronized) and add it to the map using putIfAbsent. If you managed to create the key/value pair (compare the returned object using == to the one you made; if they are the same, you were the one to add it), you got it, go, run the code. Once you're done, acquire the sync lock on your object, send a notification, release, and remove:
public void doWithLocking(String key, Runnable op) {
Object locker = new Object();
Object o = concurrentMap.putIfAbsent(key, locker);
if (o == locker) {
op.run();
synchronized (locker) {
locker.notifyAll(); // wake up everybody waiting.
concurrentMap.remove(key); // this has to be inside!
}
} else {
...
}
}
To wait until the lock is available, first acquire a lock on the locker object, THEN check if the concurrentMap still contains it. If not, you're now free to retry this operation. If it's still in, then we now wait for a notification. In any case we always just retry from scratch. Thus:
public void performWithLocking(String key, Runnable op) throws InterruptedException {
while (true) {
Object locker = new Object();
Object o = concurrentMap.putIfAbsent(key, locker);
if (o == locker) {
try {
op.run();
} finally {
// We want to lock even if the operation throws!
synchronized (locker) {
locker.notifyAll(); // wake up everybody waiting.
concurrentMap.remove(key); // this has to be inside!
}
}
return;
} else {
synchronized (o) {
if (concurrentMap.containsKey(key)) o.wait();
}
}
}
}
}
Instead of this setup where you pass the operation to execute along with the lock key, you could have tandem 'lock' and 'unlock' methods but now you run the risk of writing code that forgets to call unlock. Hence why I wouldn't advise it!
You can call this with, for example:
keyedLockSupportThingie.doWithLocking("mykey", () -> {
System.out.println("Hello, from safety!");
});
I need to create a class that has a shared-between-threads Object (lets call is SharedObject). The special thing about SharedObject is that it holds a String that will be returned in multithreaded environment, and sometimes the entire SharedObject will be written to by changing field reference to newly created object.
I do not want to make the read and write both synchronised on the same monitor because the write scenario is happening rarely while read scenario is quite common. Therefore I did the following:
public class ObjectHolder {
private volatile SharedObject sharedObject;
public String getSharedObjectString() {
if (!isObjectStillValid()) {
obtainNewSharedObject()
}
return sharedObject.getCommonString()
}
public synchronized void obtainNewSharedObject() {
/* This is in case multiple threads wait on this lock,
after first one obtains new object the others can just
use it and should not obtain a new one */
if(!isObjectStillValid()) {
sharedObject = new SharedObject(/*some parameters from somewhere*/)
}
}
}
From what I have read from documentation and on stackoverflow, the synchronized keyword will assure only one thread can access the synchronised block on the same object instance(therefore write race/multiple unnecessary writes is a non-issue) while volatile keyword on the field reference will assure the reference value is written directly to the main program memory (not cached locally).
Are there any other pitfalls I am missing?
I want to be sure that within synchronized block when sharedObject is written to, the new value of sharedObject is present for any other thread at latest when lock for obtainNewSharedObject() is released. Should this not be guaranteed, I could encounter scenarios of unnecessary writes and replacing correct values which are a big problem for this case.
I know to be absolutely safe I could just make getSharedObjectString() synchronized by itself however as stated previously I do not want to block reading if not needed.
This way reading is non-blocking, when a write scenario occurs it is blocking.
I should probably mention method isObjectStillValid() is thread independant (entirely SharedObject and System clock based) therefore a valid Thread-free check to be used for write scenarios.
Edit: Please note I could not find a similar topic on stackoverflow, but it may exist. Sorry if that is the case.
Edit2: Thank you for all the comments. Edit because apparently I cannot upvote yet (I can, but it does not show). While my solution is functional as long as isObjectStillValid is thread-safe, it can suffer from decreased performance due to multiple accesses to volatile field. I will improve it most likely using the upgraded double-checked locking solution. I will also in-depth analyse all the other possibilities mentioned here.
Why don't you use AtomicReference. It uses optimistic locking, meaning that no actual thread locking is involved. Internally it uses Compare and Swap. If you look at the implementation it uses volatile in its implementation and I would trust Doug Lea to implement it correctly :)
Apart from this, there many more ways for synchronization between lot of readers and some writers - ReadWriteLock
This looks like a classic double-checked locking pattern. While your implementation is logically correct - thanks to the use of volatile on sharedObject - it might not be the most performant.
The recommended pattern for Java 1.5 on is shown on the Wikipedia page linked.
// Works with acquire/release semantics for volatile in Java 1.5 and later
// Broken under Java 1.4 and earlier semantics for volatile
class Foo {
private volatile Helper helper;
public Helper getHelper() {
Helper localRef = helper;
if (localRef == null) {
synchronized(this) {
localRef = helper;
if (localRef == null) {
helper = localRef = new Helper();
}
}
}
return localRef;
}
// other functions and members...
}
Note the use of a localRef for accessing the helper field. This limits access to the volatile field in the simple case to a single read instead of potentially twice; once for the check and once for the return. See the Wikipedia page again, just after the recommended pattern sample.
Note the local variable "localRef", which seems unnecessary. The effect of this is that in cases where helper is already initialized (i.e., most of the time), the volatile field is only accessed once (due to "return localRef;" instead of "return helper;"), which can improve the method's overall performance by as much as 25 percent.[7]
Depending on how isObjectStillValid() accesses sharedObject, you might benefit from a similar pattern.
This sounds like the use of a ReadWriteLock would be appropiate.
The basic idea is that there can be multiple readers simultaniously or one writer exclusively. Here can you find an Example how to use it in a List implementation.
Copy paste in case the side goes down:
import java.util.*;
import java.util.concurrent.locks.*;
/**
* ReadWriteList.java
* This class demonstrates how to use ReadWriteLock to add concurrency
* features to a non-threadsafe collection
* #author www.codejava.net
*/
public class ReadWriteList<E> {
private List<E> list = new ArrayList<>();
private ReadWriteLock rwLock = new ReentrantReadWriteLock();
public ReadWriteList(E... initialElements) {
list.addAll(Arrays.asList(initialElements));
}
public void add(E element) {
Lock writeLock = rwLock.writeLock();
writeLock.lock();
try {
list.add(element);
} finally {
writeLock.unlock();
}
}
public E get(int index) {
Lock readLock = rwLock.readLock();
readLock.lock();
try {
return list.get(index);
} finally {
readLock.unlock();
}
}
public int size() {
Lock readLock = rwLock.readLock();
readLock.lock();
try {
return list.size();
} finally {
readLock.unlock();
}
}
}
A warning is showing every time I synchronize on a non-final class field. Here is the code:
public class X
{
private Object o;
public void setO(Object o)
{
this.o = o;
}
public void x()
{
synchronized (o) // synchronization on a non-final field
{
}
}
}
so I changed the coding in the following way:
public class X
{
private final Object o;
public X()
{
o = new Object();
}
public void x()
{
synchronized (o)
{
}
}
}
I am not sure the above code is the proper way to synchronize on a non-final class field. How can I synchronize a non final field?
First of all, I encourage you to really try hard to deal with concurrency issues on a higher level of abstraction, i.e. solving it using classes from java.util.concurrent such as ExecutorServices, Callables, Futures etc.
That being said, there's nothing wrong with synchronizing on a non-final field per se. You just need to keep in mind that if the object reference changes, the same section of code may be run in parallel. I.e., if one thread runs the code in the synchronized block and someone calls setO(...), another thread can run the same synchronized block on the same instance concurrently.
Synchronize on the object which you need exclusive access to (or, better yet, an object dedicated to guarding it).
It's really not a good idea - because your synchronized blocks are no longer really synchronized in a consistent way.
Assuming the synchronized blocks are meant to be ensuring that only one thread accesses some shared data at a time, consider:
Thread 1 enters the synchronized block. Yay - it has exclusive access to the shared data...
Thread 2 calls setO()
Thread 3 (or still 2...) enters the synchronized block. Eek! It think it has exclusive access to the shared data, but thread 1 is still furtling with it...
Why would you want this to happen? Maybe there are some very specialized situations where it makes sense... but you'd have to present me with a specific use case (along with ways of mitigating the sort of scenario I've given above) before I'd be happy with it.
I agree with one of John's comment: You must always use a final lock dummy while accessing a non-final variable to prevent inconsistencies in case of the variable's reference changes. So in any cases and as a first rule of thumb:
Rule#1: If a field is non-final, always use a (private) final lock dummy.
Reason #1: You hold the lock and change the variable's reference by yourself. Another thread waiting outside the synchronized lock will be able to enter the guarded block.
Reason #2: You hold the lock and another thread changes the variable's reference. The result is the same: Another thread can enter the guarded block.
But when using a final lock dummy, there is another problem: You might get wrong data, because your non-final object will only be synchronized with RAM when calling synchronize(object). So, as a second rule of thumb:
Rule#2: When locking a non-final object you always need to do both: Using a final lock dummy and the lock of the non-final object for the sake of RAM synchronisation. (The only alternative will be declaring all fields of the object as volatile!)
These locks are also called "nested locks". Note that you must call them always in the same order, otherwise you will get a dead lock:
public class X {
private final LOCK;
private Object o;
public void setO(Object o){
this.o = o;
}
public void x() {
synchronized (LOCK) {
synchronized(o){
//do something with o...
}
}
}
}
As you can see I write the two locks directly on the same line, because they always belong together. Like this, you could even do 10 nesting locks:
synchronized (LOCK1) {
synchronized (LOCK2) {
synchronized (LOCK3) {
synchronized (LOCK4) {
//entering the locked space
}
}
}
}
Note that this code won't break if you just acquire an inner lock like synchronized (LOCK3) by another threads. But it will break if you call in another thread something like this:
synchronized (LOCK4) {
synchronized (LOCK1) { //dead lock!
synchronized (LOCK3) {
synchronized (LOCK2) {
//will never enter here...
}
}
}
}
There is only one workaround around such nested locks while handling non-final fields:
Rule #2 - Alternative: Declare all fields of the object as volatile. (I won't talk here about the disadvantages of doing this, e.g. preventing any storage in x-level caches even for reads, aso.)
So therefore aioobe is quite right: Just use java.util.concurrent. Or begin to understand everything about synchronisation and do it by yourself with nested locks. ;)
For more details why synchronisation on non-final fields breaks, have a look into my test case: https://stackoverflow.com/a/21460055/2012947
And for more details why you need synchronized at all due to RAM and caches have a look here: https://stackoverflow.com/a/21409975/2012947
I'm not really seeing the correct answer here, that is, It's perfectly alright to do it.
I'm not even sure why it's a warning, there is nothing wrong with it. The JVM makes sure that you get some valid object back (or null) when you read a value, and you can synchronize on any object.
If you plan on actually changing the lock while it's in use (as opposed to e.g. changing it from an init method, before you start using it), you have to make the variable that you plan to change volatile. Then all you need to do is to synchronize on both the old and the new object, and you can safely change the value
public volatile Object lock;
...
synchronized (lock) {
synchronized (newObject) {
lock = newObject;
}
}
There. It's not complicated, writing code with locks (mutexes) is actally quite easy. Writing code without them (lock free code) is what's hard.
EDIT: So this solution (as suggested by Jon Skeet) might have an issue with atomicity of implementation of "synchronized(object){}" while object reference is changing. I asked separately and according to Mr. erickson it is not thread safe - see: Is entering synchronized block atomic?. So take it as example how to NOT do it - with links why ;)
See the code how it would work if synchronised() would be atomic:
public class Main {
static class Config{
char a='0';
char b='0';
public void log(){
synchronized(this){
System.out.println(""+a+","+b);
}
}
}
static Config cfg = new Config();
static class Doer extends Thread {
char id;
Doer(char id) {
this.id = id;
}
public void mySleep(long ms){
try{Thread.sleep(ms);}catch(Exception ex){ex.printStackTrace();}
}
public void run() {
System.out.println("Doer "+id+" beg");
if(id == 'X'){
synchronized (cfg){
cfg.a=id;
mySleep(1000);
// do not forget to put synchronize(cfg) over setting new cfg - otherwise following will happend
// here it would be modifying different cfg (cos Y will change it).
// Another problem would be that new cfg would be in parallel modified by Z cos synchronized is applied on new object
cfg.b=id;
}
}
if(id == 'Y'){
mySleep(333);
synchronized(cfg) // comment this and you will see inconsistency in log - if you keep it I think all is ok
{
cfg = new Config(); // introduce new configuration
// be aware - don't expect here to be synchronized on new cfg!
// Z might already get a lock
}
}
if(id == 'Z'){
mySleep(666);
synchronized (cfg){
cfg.a=id;
mySleep(100);
cfg.b=id;
}
}
System.out.println("Doer "+id+" end");
cfg.log();
}
}
public static void main(String[] args) throws InterruptedException {
Doer X = new Doer('X');
Doer Y = new Doer('Y');
Doer Z = new Doer('Z');
X.start();
Y.start();
Z.start();
}
}
AtomicReference suits for your requirement.
From java documentation about atomic package:
A small toolkit of classes that support lock-free thread-safe programming on single variables. In essence, the classes in this package extend the notion of volatile values, fields, and array elements to those that also provide an atomic conditional update operation of the form:
boolean compareAndSet(expectedValue, updateValue);
Sample code:
String initialReference = "value 1";
AtomicReference<String> someRef =
new AtomicReference<String>(initialReference);
String newReference = "value 2";
boolean exchanged = someRef.compareAndSet(initialReference, newReference);
System.out.println("exchanged: " + exchanged);
In above example, you replace String with your own Object
Related SE question:
When to use AtomicReference in Java?
If o never changes for the lifetime of an instance of X, the second version is better style irrespective of whether synchronization is involved.
Now, whether there's anything wrong with the first version is impossible to answer without knowing what else is going on in that class. I would tend to agree with the compiler that it does look error-prone (I won't repeat what the others have said).
Just adding my two cents: I had this warning when I used component that is instantiated through designer, so it's field cannot really be final, because constructor cannot takes parameters. In other words, I had quasi-final field without the final keyword.
I think that's why it is just warning: you are probably doing something wrong, but it might be right as well.
Can any one tell me the advantage of synchronized method over synchronized block with an example?
Can anyone tell me the advantage of the synchronized method over the synchronized block with an example? Thanks.
There is not a clear advantage of using synchronized method over the block.
Perhaps the only one ( but I wouldn't call it an advantage ) is you don't need to include the object reference this.
Method:
public synchronized void method() { // blocks "this" from here....
...
...
...
} // to here
Block:
public void method() {
synchronized( this ) { // blocks "this" from here ....
....
....
....
} // to here...
}
See? No advantage at all.
Blocks do have advantages over methods though, mostly in flexibility because you can use another object as lock whereas syncing the method would lock the entire object.
Compare:
// locks the whole object
...
private synchronized void someInputRelatedWork() {
...
}
private synchronized void someOutputRelatedWork() {
...
}
vs.
// Using specific locks
Object inputLock = new Object();
Object outputLock = new Object();
private void someInputRelatedWork() {
synchronized(inputLock) {
...
}
}
private void someOutputRelatedWork() {
synchronized(outputLock) {
...
}
}
Also if the method grows you can still keep the synchronized section separated:
private void method() {
... code here
... code here
... code here
synchronized( lock ) {
... very few lines of code here
}
... code here
... code here
... code here
... code here
}
The only real difference is that a synchronized block can choose which object it synchronizes on. A synchronized method can only use 'this' (or the corresponding Class instance for a synchronized class method). For example, these are semantically equivalent:
synchronized void foo() {
...
}
void foo() {
synchronized (this) {
...
}
}
The latter is more flexible since it can compete for the associated lock of any object, often a member variable. It's also more granular because you could have concurrent code executing before and after the block but still within the method. Of course, you could just as easily use a synchronized method by refactoring the concurrent code into separate non-synchronized methods. Use whichever makes the code more comprehensible.
Synchronized Method
Pros:
Your IDE can indicate the synchronized methods.
The syntax is more compact.
Forces to split the synchronized blocks to separate methods.
Cons:
Synchronizes to this and so makes it possible to outsiders to synchronize to it too.
It is harder to move code outside the synchronized block.
Synchronized block
Pros:
Allows using a private variable for the lock and so forcing the lock to stay inside the class.
Synchronized blocks can be found by searching references to the variable.
Cons:
The syntax is more complicated and so makes the code harder to read.
Personally I prefer using synchronized methods with classes focused only to the thing needing synchronization. Such class should be as small as possible and so it should be easy to review the synchronization. Others shouldn't need to care about synchronization.
The main difference is that if you use a synchronized block you may lock on an object other than this which allows to be much more flexible.
Assume you have a message queue and multiple message producers and consumers. We don't want producers to interfere with each other, but the consumers should be able to retrieve messages without having to wait for the producers.
So we just create an object
Object writeLock = new Object();
And from now on every time a producers wants to add a new message we just lock on that:
synchronized(writeLock){
// do something
}
So consumers may still read, and producers will be locked.
Synchronized method
Synchronized methods have two effects.
First, when one thread is executing a synchronized method for an object, all other threads that invoke synchronized methods for the same object block (suspend execution) until the first thread is done with the object.
Second, when a synchronized method exits, it automatically establishes a happens-before relationship with any subsequent invocation of a synchronized method for the same object. This guarantees that changes to the state of the object are visible to all threads.
Note that constructors cannot be synchronized — using the synchronized keyword with a constructor is a syntax error. Synchronizing constructors doesn't make sense, because only the thread that creates an object should have access to it while it is being constructed.
Synchronized Statement
Unlike synchronized methods, synchronized statements must specify the object that provides the intrinsic lock: Most often I use this to synchronize access to a list or map but I don't want to block access to all methods of the object.
Q: Intrinsic Locks and Synchronization
Synchronization is built around an internal entity known as the intrinsic lock or monitor lock. (The API specification often refers to this entity simply as a "monitor.") Intrinsic locks play a role in both aspects of synchronization: enforcing exclusive access to an object's state and establishing happens-before relationships that are essential to visibility.
Every object has an intrinsic lock associated with it. By convention, a thread that needs exclusive and consistent access to an object's fields has to acquire the object's intrinsic lock before accessing them, and then release the intrinsic lock when it's done with them. A thread is said to own the intrinsic lock between the time it has acquired the lock and released the lock. As long as a thread owns an intrinsic lock, no other thread can acquire the same lock. The other thread will block when it attempts to acquire the lock.
package test;
public class SynchTest implements Runnable {
private int c = 0;
public static void main(String[] args) {
new SynchTest().test();
}
public void test() {
// Create the object with the run() method
Runnable runnable = new SynchTest();
Runnable runnable2 = new SynchTest();
// Create the thread supplying it with the runnable object
Thread thread = new Thread(runnable,"thread-1");
Thread thread2 = new Thread(runnable,"thread-2");
// Here the key point is passing same object, if you pass runnable2 for thread2,
// then its not applicable for synchronization test and that wont give expected
// output Synchronization method means "it is not possible for two invocations
// of synchronized methods on the same object to interleave"
// Start the thread
thread.start();
thread2.start();
}
public synchronized void increment() {
System.out.println("Begin thread " + Thread.currentThread().getName());
System.out.println(this.hashCode() + "Value of C = " + c);
// If we uncomment this for synchronized block, then the result would be different
// synchronized(this) {
for (int i = 0; i < 9999999; i++) {
c += i;
}
// }
System.out.println("End thread " + Thread.currentThread().getName());
}
// public synchronized void decrement() {
// System.out.println("Decrement " + Thread.currentThread().getName());
// }
public int value() {
return c;
}
#Override
public void run() {
this.increment();
}
}
Cross check different outputs with synchronized method, block and without synchronization.
Note: static synchronized methods and blocks work on the Class object.
public class MyClass {
// locks MyClass.class
public static synchronized void foo() {
// do something
}
// similar
public static void foo() {
synchronized(MyClass.class) {
// do something
}
}
}
When java compiler converts your source code to byte code, it handles synchronized methods and synchronized blocks very differently.
When the JVM executes a synchronized method, the executing thread identifies that the method's method_info structure has the ACC_SYNCHRONIZED flag set, then it automatically acquires the object's lock, calls the method, and releases the lock. If an exception occurs, the thread automatically releases the lock.
Synchronizing a method block, on the other hand, bypasses the JVM's built-in support for acquiring an object's lock and exception handling and requires that the functionality be explicitly written in byte code. If you read the byte code for a method with a synchronized block, you will see more than a dozen additional operations to manage this functionality.
This shows calls to generate both a synchronized method and a synchronized block:
public class SynchronizationExample {
private int i;
public synchronized int synchronizedMethodGet() {
return i;
}
public int synchronizedBlockGet() {
synchronized( this ) {
return i;
}
}
}
The synchronizedMethodGet() method generates the following byte code:
0: aload_0
1: getfield
2: nop
3: iconst_m1
4: ireturn
And here's the byte code from the synchronizedBlockGet() method:
0: aload_0
1: dup
2: astore_1
3: monitorenter
4: aload_0
5: getfield
6: nop
7: iconst_m1
8: aload_1
9: monitorexit
10: ireturn
11: astore_2
12: aload_1
13: monitorexit
14: aload_2
15: athrow
One significant difference between synchronized method and block is that, Synchronized block generally reduce scope of lock. As scope of lock is inversely proportional to performance, its always better to lock only critical section of code. One of the best example of using synchronized block is double checked locking in Singleton pattern where instead of locking whole getInstance() method we only lock critical section of code which is used to create Singleton instance. This improves performance drastically because locking is only required one or two times.
While using synchronized methods, you will need to take extra care if you mix both static synchronized and non-static synchronized methods.
Most often I use this to synchronize access to a list or map but I don't want to block access to all methods of the object.
In the following code one thread modifying the list will not block waiting for a thread that is modifying the map. If the methods were synchronized on the object then each method would have to wait even though the modifications they are making would not conflict.
private List<Foo> myList = new ArrayList<Foo>();
private Map<String,Bar) myMap = new HashMap<String,Bar>();
public void put( String s, Bar b ) {
synchronized( myMap ) {
myMap.put( s,b );
// then some thing that may take a while like a database access or RPC or notifying listeners
}
}
public void hasKey( String s, ) {
synchronized( myMap ) {
myMap.hasKey( s );
}
}
public void add( Foo f ) {
synchronized( myList ) {
myList.add( f );
// then some thing that may take a while like a database access or RPC or notifying listeners
}
}
public Thing getMedianFoo() {
Foo med = null;
synchronized( myList ) {
Collections.sort(myList);
med = myList.get(myList.size()/2);
}
return med;
}
With synchronized blocks, you can have multiple synchronizers, so that multiple simultaneous but non-conflicting things can go on at the same time.
Synchronized methods can be checked using reflection API. This can be useful for testing some contracts, such as all methods in model are synchronized.
The following snippet prints all the synchronized methods of Hashtable:
for (Method m : Hashtable.class.getMethods()) {
if (Modifier.isSynchronized(m.getModifiers())) {
System.out.println(m);
}
}
Important note on using the synchronized block: careful what you use as lock object!
The code snippet from user2277816 above illustrates this point in that a reference to a string literal is used as locking object.
Realize that string literals are automatically interned in Java and you should begin to see the problem: every piece of code that synchronizes on the literal "lock", shares the same lock! This can easily lead to deadlocks with completely unrelated pieces of code.
It is not just String objects that you need to be careful with. Boxed primitives are also a danger, since autoboxing and the valueOf methods can reuse the same objects, depending on the value.
For more information see:
https://www.securecoding.cert.org/confluence/display/java/LCK01-J.+Do+not+synchronize+on+objects+that+may+be+reused
Often using a lock on a method level is too rude. Why lock up a piece of code that does not access any shared resources by locking up an entire method. Since each object has a lock, you can create dummy objects to implement block level synchronization.
The block level is more efficient because it does not lock the whole method.
Here some example
Method Level
class MethodLevel {
//shared among threads
SharedResource x, y ;
public void synchronized method1() {
//multiple threads can't access
}
public void synchronized method2() {
//multiple threads can't access
}
public void method3() {
//not synchronized
//multiple threads can access
}
}
Block Level
class BlockLevel {
//shared among threads
SharedResource x, y ;
//dummy objects for locking
Object xLock = new Object();
Object yLock = new Object();
public void method1() {
synchronized(xLock){
//access x here. thread safe
}
//do something here but don't use SharedResource x, y
// because will not be thread-safe
synchronized(xLock) {
synchronized(yLock) {
//access x,y here. thread safe
}
}
//do something here but don't use SharedResource x, y
//because will not be thread-safe
}//end of method1
}
[Edit]
For Collection like Vector and Hashtable they are synchronized when ArrayList or HashMap are not and you need set synchronized keyword or invoke Collections synchronized method:
Map myMap = Collections.synchronizedMap (myMap); // single lock for the entire map
List myList = Collections.synchronizedList (myList); // single lock for the entire list
The only difference : synchronized blocks allows granular locking unlike synchronized method
Basically synchronized block or methods have been used to write thread safe code by avoiding memory inconsistency errors.
This question is very old and many things have been changed during last 7 years.
New programming constructs have been introduced for thread safety.
You can achieve thread safety by using advanced concurrency API instead of synchronied blocks. This documentation page provides good programming constructs to achieve thread safety.
Lock Objects support locking idioms that simplify many concurrent applications.
Executors define a high-level API for launching and managing threads. Executor implementations provided by java.util.concurrent provide thread pool management suitable for large-scale applications.
Concurrent Collections make it easier to manage large collections of data, and can greatly reduce the need for synchronization.
Atomic Variables have features that minimize synchronization and help avoid memory consistency errors.
ThreadLocalRandom (in JDK 7) provides efficient generation of pseudorandom numbers from multiple threads.
Better replacement for synchronized is ReentrantLock, which uses Lock API
A reentrant mutual exclusion Lock with the same basic behavior and semantics as the implicit monitor lock accessed using synchronized methods and statements, but with extended capabilities.
Example with locks:
class X {
private final ReentrantLock lock = new ReentrantLock();
// ...
public void m() {
lock.lock(); // block until condition holds
try {
// ... method body
} finally {
lock.unlock()
}
}
}
Refer to java.util.concurrent and java.util.concurrent.atomic packages too for other programming constructs.
Refer to this related question too:
Synchronization vs Lock
Synchronized method is used for lock all the objects
Synchronized block is used to lock specific object
In general these are mostly the same other than being explicit about the object's monitor that's being used vs the implicit this object. One downside of synchronized methods that I think is sometimes overlooked is that in using the "this" reference to synchronize on you are leaving open the possibility of external objects locking on the same object. That can be a very subtle bug if you run into it. Synchronizing on an internal explicit Object or other existing field can avoid this issue, completely encapsulating the synchronization.
As already said here synchronized block can use user-defined variable as lock object, when synchronized function uses only "this". And of course you can manipulate with areas of your function which should be synchronized.
But everyone says that no difference between synchronized function and block which covers whole function using "this" as lock object. That is not true, difference is in byte code which will be generated in both situations. In case of synchronized block usage should be allocated local variable which holds reference to "this". And as result we will have a little bit larger size for function (not relevant if you have only few number of functions).
More detailed explanation of the difference you can find here:
http://www.artima.com/insidejvm/ed2/threadsynchP.html
In case of synchronized methods, lock will be acquired on an Object. But if you go with synchronized block you have an option to specify an object on which the lock will be acquired.
Example :
Class Example {
String test = "abc";
// lock will be acquired on String test object.
synchronized (test) {
// do something
}
lock will be acquired on Example Object
public synchronized void testMethod() {
// do some thing
}
}
I know this is an old question, but with my quick read of the responses here, I didn't really see anyone mention that at times a synchronized method may be the wrong lock.
From Java Concurrency In Practice (pg. 72):
public class ListHelper<E> {
public List<E> list = Collections.syncrhonizedList(new ArrayList<>());
...
public syncrhonized boolean putIfAbsent(E x) {
boolean absent = !list.contains(x);
if(absent) {
list.add(x);
}
return absent;
}
The above code has the appearance of being thread-safe. However, in reality it is not. In this case the lock is obtained on the instance of the class. However, it is possible for the list to be modified by another thread not using that method. The correct approach would be to use
public boolean putIfAbsent(E x) {
synchronized(list) {
boolean absent = !list.contains(x);
if(absent) {
list.add(x);
}
return absent;
}
}
The above code would block all threads trying to modify list from modifying the list until the synchronized block has completed.
As a practical matter, the advantage of synchronized methods over synchronized blocks is that they are more idiot-resistant; because you can't choose an arbitrary object to lock on, you can't misuse the synchronized method syntax to do stupid things like locking on a string literal or locking on the contents of a mutable field that gets changed out from under the threads.
On the other hand, with synchronized methods you can't protect the lock from getting acquired by any thread that can get a reference to the object.
So using synchronized as a modifier on methods is better at protecting your cow-orkers from hurting themselves, while using synchronized blocks in conjunction with private final lock objects is better at protecting your own code from the cow-orkers.
From a Java specification summary:
http://www.cs.cornell.edu/andru/javaspec/17.doc.html
The synchronized statement (§14.17) computes a reference to an object;
it then attempts to perform a lock action on that object and does not
proceed further until the lock action has successfully completed. ...
A synchronized method (§8.4.3.5) automatically performs a lock action
when it is invoked; its body is not executed until the lock action has
successfully completed. If the method is an instance method, it
locks the lock associated with the instance for which it was invoked
(that is, the object that will be known as this during execution of
the body of the method). If the method is static, it locks the
lock associated with the Class object that represents the class in
which the method is defined. ...
Based on these descriptions, I would say most previous answers are correct, and a synchronized method might be particularly useful for static methods, where you would otherwise have to figure out how to get the "Class object that represents the class in which the method was defined."
Edit: I originally thought these were quotes of the actual Java spec. Clarified that this page is just a summary/explanation of the spec
TLDR; Neither use the synchronized modifier nor the synchronized(this){...} expression but synchronized(myLock){...} where myLock is a final instance field holding a private object.
The difference between using the synchronized modifier on the method declaration and the synchronized(..){ } expression in the method body are this:
The synchronized modifier specified on the method's signature
is visible in the generated JavaDoc,
is programmatically determinable via reflection when testing a method's modifier for Modifier.SYNCHRONIZED,
requires less typing and indention compared to synchronized(this) { .... }, and
(depending on your IDE) is visible in the class outline and code completion,
uses the this object as lock when declared on non-static method or the enclosing class when declared on a static method.
The synchronized(...){...} expression allows you
to only synchronize the execution of parts of a method's body,
to be used within a constructor or a (static) initialization block,
to choose the lock object which controls the synchronized access.
However, using the synchronized modifier or synchronized(...) {...} with this as the lock object (as in synchronized(this) {...}), have the same disadvantage. Both use it's own instance as the lock object to synchronize on. This is dangerous because not only the object itself but any other external object/code that holds a reference to that object can also use it as a synchronization lock with potentially severe side effects (performance degradation and deadlocks).
Therefore best practice is to neither use the synchronized modifier nor the synchronized(...) expression in conjunction with this as lock object but a lock object private to this object. For example:
public class MyService {
private final lock = new Object();
public void doThis() {
synchronized(lock) {
// do code that requires synchronous execution
}
}
public void doThat() {
synchronized(lock) {
// do code that requires synchronous execution
}
}
}
You can also use multiple lock objects but special care needs to be taken to ensure this does not result in deadlocks when used nested.
public class MyService {
private final lock1 = new Object();
private final lock2 = new Object();
public void doThis() {
synchronized(lock1) {
synchronized(lock2) {
// code here is guaranteed not to be executes at the same time
// as the synchronized code in doThat() and doMore().
}
}
public void doThat() {
synchronized(lock1) {
// code here is guaranteed not to be executes at the same time
// as the synchronized code in doThis().
// doMore() may execute concurrently
}
}
public void doMore() {
synchronized(lock2) {
// code here is guaranteed not to be executes at the same time
// as the synchronized code in doThis().
// doThat() may execute concurrently
}
}
}
I suppose this question is about the difference between Thread Safe Singleton and Lazy initialization with Double check locking. I always refer to this article when I need to implement some specific singleton.
Well, this is a Thread Safe Singleton:
// Java program to create Thread Safe
// Singleton class
public class GFG
{
// private instance, so that it can be
// accessed by only by getInstance() method
private static GFG instance;
private GFG()
{
// private constructor
}
//synchronized method to control simultaneous access
synchronized public static GFG getInstance()
{
if (instance == null)
{
// if instance is null, initialize
instance = new GFG();
}
return instance;
}
}
Pros:
Lazy initialization is possible.
It is thread safe.
Cons:
getInstance() method is synchronized so it causes slow performance as multiple threads can’t access it simultaneously.
This is a Lazy initialization with Double check locking:
// Java code to explain double check locking
public class GFG
{
// private instance, so that it can be
// accessed by only by getInstance() method
private static GFG instance;
private GFG()
{
// private constructor
}
public static GFG getInstance()
{
if (instance == null)
{
//synchronized block to remove overhead
synchronized (GFG.class)
{
if(instance==null)
{
// if instance is null, initialize
instance = new GFG();
}
}
}
return instance;
}
}
Pros:
Lazy initialization is possible.
It is also thread safe.
Performance reduced because of synchronized keyword is overcome.
Cons:
First time, it can affect performance.
As cons. of double check locking method is bearable so it can be
used for high performance multi-threaded applications.
Please refer to this article for more details:
https://www.geeksforgeeks.org/java-singleton-design-pattern-practices-examples/
Synchronizing with threads.
1) NEVER use synchronized(this) in a thread it doesn't work. Synchronizing with (this) uses the current thread as the locking thread object. Since each thread is independent of other threads, there is NO coordination of synchronization.
2) Tests of code show that in Java 1.6 on a Mac the method synchronization does not work.
3) synchronized(lockObj) where lockObj is a common shared object of all threads synchronizing on it will work.
4) ReenterantLock.lock() and .unlock() work. See Java tutorials for this.
The following code shows these points. It also contains the thread-safe Vector which would be substituted for the ArrayList, to show that many threads adding to a Vector do not lose any information, while the same with an ArrayList can lose information.
0) Current code shows loss of information due to race conditions
A) Comment the current labeled A line, and uncomment the A line above it, then run, method loses data but it shouldn't.
B) Reverse step A, uncomment B and // end block }. Then run to see results no loss of data
C) Comment out B, uncomment C. Run, see synchronizing on (this) loses data, as expected.
Don't have time to complete all the variations, hope this helps.
If synchronizing on (this), or the method synchronization works, please state what version of Java and OS you tested. Thank you.
import java.util.*;
/** RaceCondition - Shows that when multiple threads compete for resources
thread one may grab the resource expecting to update a particular
area but is removed from the CPU before finishing. Thread one still
points to that resource. Then thread two grabs that resource and
completes the update. Then thread one gets to complete the update,
which over writes thread two's work.
DEMO: 1) Run as is - see missing counts from race condition, Run severa times, values change
2) Uncomment "synchronized(countLock){ }" - see counts work
Synchronized creates a lock on that block of code, no other threads can
execute code within a block that another thread has a lock.
3) Comment ArrayList, unComment Vector - See no loss in collection
Vectors work like ArrayList, but Vectors are "Thread Safe"
May use this code as long as attribution to the author remains intact.
/mf
*/
public class RaceCondition {
private ArrayList<Integer> raceList = new ArrayList<Integer>(); // simple add(#)
// private Vector<Integer> raceList = new Vector<Integer>(); // simple add(#)
private String countLock="lock"; // Object use for locking the raceCount
private int raceCount = 0; // simple add 1 to this counter
private int MAX = 10000; // Do this 10,000 times
private int NUM_THREADS = 100; // Create 100 threads
public static void main(String [] args) {
new RaceCondition();
}
public RaceCondition() {
ArrayList<Thread> arT = new ArrayList<Thread>();
// Create thread objects, add them to an array list
for( int i=0; i<NUM_THREADS; i++){
Thread rt = new RaceThread( ); // i );
arT.add( rt );
}
// Start all object at once.
for( Thread rt : arT ){
rt.start();
}
// Wait for all threads to finish before we can print totals created by threads
for( int i=0; i<NUM_THREADS; i++){
try { arT.get(i).join(); }
catch( InterruptedException ie ) { System.out.println("Interrupted thread "+i); }
}
// All threads finished, print the summary information.
// (Try to print this informaiton without the join loop above)
System.out.printf("\nRace condition, should have %,d. Really have %,d in array, and count of %,d.\n",
MAX*NUM_THREADS, raceList.size(), raceCount );
System.out.printf("Array lost %,d. Count lost %,d\n",
MAX*NUM_THREADS-raceList.size(), MAX*NUM_THREADS-raceCount );
} // end RaceCondition constructor
class RaceThread extends Thread {
public void run() {
for ( int i=0; i<MAX; i++){
try {
update( i );
} // These catches show when one thread steps on another's values
catch( ArrayIndexOutOfBoundsException ai ){ System.out.print("A"); }
catch( OutOfMemoryError oome ) { System.out.print("O"); }
}
}
// so we don't lose counts, need to synchronize on some object, not primitive
// Created "countLock" to show how this can work.
// Comment out the synchronized and ending {, see that we lose counts.
// public synchronized void update(int i){ // use A
public void update(int i){ // remove this when adding A
// synchronized(countLock){ // or B
// synchronized(this){ // or C
raceCount = raceCount + 1;
raceList.add( i ); // use Vector
// } // end block for B or C
} // end update
} // end RaceThread inner class
} // end RaceCondition outter class