say I have the following two classes.
public class Foo {
private volatile Integer x = 0;
private volatile boolean istrue = false;
public void setInt(int x) {
this.x = x;
}
public void setBoolean(boolean istrue) {
this.istrue = istrue;
}
public Integer getInt() {
return x;
}
}
vs
public class Bar {
private volatile AtomicInteger x = 0;
private volatile AtomicBoolean istrue = false;
public void setInt(int x) {
this.x.set(x);
}
public void setBoolean(boolean istrue) {
this.istrue.set(istrue);
}
public Integer getInt() {
return this.x.get();
}
}
Assume multiple threads can access Foo or Bar. Are both classes thread safe? what is the difference between the two classes really?
In the current example you have single assignment statements in the methods. I think you need to come up with a better example. Because the assignment will be executed in a single instruction. Which makes both the implementation thread safe. Even though, it is possible that, two different threads see different values of the int when they access them, because by the time, other thread can potentially reset (set) to a different value.
Check this out: What is thread Safe in java?
Both classes are thread safe as written. The difference between use of a volatile variable and the java.util.concurrent.atomic classes is that the atomic classes permit more complex operations, such as compare and set, to be performed atomically. With just direct access to a volatile variable, there could be a thread context switch between the compare and the set, so you can't rely on the result of the compare still being valid when the set is done.
Both classes are thread safe. But that is not your issue here.
Foo: reads and writes to the fields are atomic. But more complex operations such as i++ are not. i++ translates to i = i + 1, which decomposes to a read and a write, so there could a thread context switch in between making the entire operation not atomic.
Bar: the volatile makes the access to the Atomic fields themselves atomic, so you can reassign the fields with new references in an atomic ways. The operations on the fields like compareAndSet or inc however are atomic. Question is, do you need atomic write access to the fields or just atomicity on the operations? As the AtomicXXX type are just containers for for values, you typically don't reassign the variable, but reassign values, which is atomic.
So this should be sufficient:
private final AtomicInteger x = new AtomicInteger(0);
private final AtomicBoolean istrue = new AtomicBoolean(false);
Related
Java Code:
public class IncreaseTest {
public static int value = 0;
public synchronized int increment() {
return value++;
}
}
Is method increment() thread-safe? Do I have to add the modifier keyword volatile as follows：
public static volatile int value = 0;
This code is not thread-safe. The instance method will synchronize on an instance, if you have multiple instances they will not use the same monitor and therefor the updates can interleave.
You either need to remove the static from the value field or add static to the increment() method.
Also, as you have made value public, there is the additional problem that value can be changed or read outside of this method without using synchronisation which could result in reading old values.
So changing your code to the following will make it thread-safe:
public class IncreaseTest {
private int value = 0;
public synchronized int increment() {
return value++;
}
}
You should probably use atomicvars
If you are using this method in two threads then you do need the volatile keyword. Without it, another thread may not get the most up to date value. (C#)
I do not think this is thread safe since the static variable is public and can be accessed by other threads in a non-thread safe manner. In order to be thread safe you must declare the variable as follows:
public static volatile int value;
Now value being volatile, will be accessed in a synchronized block.
In the book "Java Concurrency in Practice" the following code is attached:
public class Counter {
private long value = 0;
public synchronized long getValue() {
return value;
}
public synchronized void increment() {
++value;
}
}
Here is the value field with type long, as well as synchronized read and increment methods.
Do I understand correctly that the synchronization of the read method is needed only for the long and double types, because they are not atomic? And if the field value is of type int, synchronization of the read method will be unnecessary?
You need to synchronize the getValue method.
In Java, the synchronized keyword gives you both visibility and atomicity guarantees. As a rule of thumb, you should remember that the compiler developers and CPU vendors are free to do optimizations such as memory reordering or caching variables in registers as long as the behavior of the optimized program remains unchanged in a single-threaded environment.
Now let's see what happens when different threads call the getValue and increment methods, and the getValue is no longer synchronized. From the perspective of JVM, any thread that only reads the value and never updates it through the increment method is effectively always reading the same constant value. So inside the reader thread, JVM might choose to save the variable in a register and never read it from memory again!
As the comments have suggested, the synchronized keyword can be removed by changing the type of value from an int to AtomicInteger. But even if you don't want to use an AtomicInteger, you can rewrite your class like this:
public class Counter {
private volatile int value = 0;
public int getValue() {
return value;
}
public synchronized void increment() {
++value;
}
}
The volatile keyword will ensure visibility, and the reads no longer require any locking. I suggest checking these references to learn more about Java Memory Model:
https://jenkov.com/tutorials/java-concurrency/index.html
https://docs.oracle.com/javase/specs/jls/se17/html/jls-17.html
... without additional synchronization ? The Tree class below is meant to be accessed by multiple threads (it is a singleton but not implemented via an enum)
class Tree {
private volatile Node root;
Tree() {
root = new Node();
// the threads are spawned _after_ the tree is constructed
}
private final class Node {
short numOfKeys;
}
}
Will updates to the numOfKeys field be visible to reader threads without any explicit synchronization (notice that both readers and writers have to acquire an instance of ReentrantReadWriteLock - same instance for each node - but barring that) ? If not would making numOfKeys volatile suffice ?
Is changing the root as simple as root = new Node() (only a writer thread would change the root, apart from the main thread which calls the Tree constructor)
Related:
multiple fields: volatile or AtomicReference?
Is volatile enough for changing reference to a list?
Are mutable fields in a POJO object threadsafe if stored in a concurrentHashMap?
Using volatile keyword with mutable object
EDIT: interested in post Java 5 semantics
No.
Placing a reference to an object in a volatile field does not affect the object itself in any way.
Once you load the reference to the object from the volatile field, you have an object no different from any other object, and the volatility has no further effect.
The are two question. Let's start with the second.
Assigning newly constructed objects to volatile variables works nicely. Every thread, that reads the volatile variable, will see a fully constructed object. There is no need for further synchronization. This pattern is usually seen in combination with immutable types.
class Tree {
private volatile Node node;
public void update() {
node = new Node(...);
}
public Node get() {
return node;
}
}
Regarding the first question. You can use volatile variables to synchronize access to non-volatile variable. The following listing shows an example. Imagine that the two variables are initialized as shown, and that the two methods are executed concurrently. It is guaranteed, that if the second thread sees the update to foo, it will also see the update to bar.
volatile int foo = 0;
int bar = 0;
void thread1() {
bar = 1;
foo = 1; // write to volatile variable
}
void thread2() {
if (foo == 1) { // read from volatile variable
int r = bar; // r == 1
}
}
However, your example is different. The reading and writing might look as follows. In contrast to the above example, both threads read from the volatile variable. However, read operations on volatile variables do not synchronize with each other.
void thread1() {
Node temp = root; // read from volatile variable
temp.numOfKeys = 1;
}
void thread2() {
Node temp = root; // read from volatile variable
int r = temp.numOfKeys;
}
In other words: If thread A writes to a volatile variable x and thread B reads the value written to x, then after the read operation, thread B will see all write operations of thread A, that occurred before the write to x. But without a write operation to a volatile variable, there is no effect on updates to other variables.
That sounds more complicated than it actually is. Actually, there is only one rule to consider, which you can find in JLS8 §17.4.5:
[..] If all sequentially consistent executions are free of data races, [..] then all executions of the program will appear to be sequentially consistent.
Simply put, a data race exists if two threads can access the same variable at the same time, at least one operation is a write operation, and the variable is non-volatile. Data races can be eliminated by declaring shared variables as volatile. Without data races, there is no problem with visibility of updates.
Below is the code snippet from concurrency in practice.
class OneValueCache {
private final BigInteger lastNumber;
private final BigInteger[] lastFactors;
public OneValueCache(BigInteger i,BigInteger[] factors) {
lastNumber = i;
lastFactors = Arrays.copyOf(factors, factors.length);
}
public BigInteger[] getFactors(BigInteger i) {
if (lastNumber == null || !lastNumber.equals(i))
return null;
else
return Arrays.copyOf(lastFactors, lastFactors.length);
}
}
//Volatile is not enough to make VolatileCachedFactorizer thread safe? Why we need final specifier in OneValueCache.
public class VolatileCachedFactorizer implements Servlet {
private volatile OneValueCache cache = new OneValueCache(null, null);
//Servlet service method.
public void service(ServletRequest req, ServletResponse resp) {
BigInteger i = extractFromRequest(req);
BigInteger[] factors = cache.getFactors(i);
//Check factors are null or not.
if (factors == null) {
factors = factor(i);
cache = new OneValueCache(i, factors);
}
encodeIntoResponse(resp, factors);
}
}
What is the use of declaring fields as final in OneValueCache. "volatile OneValueCache cache" makes sure that object is visible to all the other threads and i assume that writes before volatile write is visible to all the other threads.
Final fields make OneValueCache immutable, thereby making it thread safe. They also have special semantics defined by the JLS, in particular, any thread will be able to see the correctly constructed object with the final fields initialized to its only correct value.
If this was not the case, and the fields happened to not be final, other threads might not be able to see the changes, even made in the constructor, because without final fields, there are no construction safety guarantees.
JCIP explains that OneValueCache is only an immutable reference class used to hold to the two bits of data. This is safer than updating two fields in the method, as it is not atomic. Then, OneValueCache is made volatile in the servlet because it needs to be changed, but is an atomic assignment, so no synchronization is needed.
They are two different things.
Generally,
volatile --> Creates a memory barrier which enforces the data in the cache to be flushed and forces data to be read from the main memory. So, all threads can always get the updated data for this particular field.
final -->
for primitives --> specifies that the value cannot change
for non-primitives --> The references cannot change (i.e, reference cannot point to another object).
For an object / field to be immutable, you need to ensure that it is transitively accessible by final fields and a reference of it doesn't escape.
PS : final and immutability are two different concepts. So, if you've heard of immutability, please understand that it is different from final.
It looks like the intention of OneValueCache class is to be immutable so declaring the values to be final just guarantees that at some later stage a programmer doesnt try to extend the class and overwrite the values.
... without additional synchronization ? The Tree class below is meant to be accessed by multiple threads (it is a singleton but not implemented via an enum)
class Tree {
private volatile Node root;
Tree() {
root = new Node();
// the threads are spawned _after_ the tree is constructed
}
private final class Node {
short numOfKeys;
}
}
Will updates to the numOfKeys field be visible to reader threads without any explicit synchronization (notice that both readers and writers have to acquire an instance of ReentrantReadWriteLock - same instance for each node - but barring that) ? If not would making numOfKeys volatile suffice ?
Is changing the root as simple as root = new Node() (only a writer thread would change the root, apart from the main thread which calls the Tree constructor)
Related:
multiple fields: volatile or AtomicReference?
Is volatile enough for changing reference to a list?
Are mutable fields in a POJO object threadsafe if stored in a concurrentHashMap?
Using volatile keyword with mutable object
EDIT: interested in post Java 5 semantics
No.
Placing a reference to an object in a volatile field does not affect the object itself in any way.
Once you load the reference to the object from the volatile field, you have an object no different from any other object, and the volatility has no further effect.
The are two question. Let's start with the second.
Assigning newly constructed objects to volatile variables works nicely. Every thread, that reads the volatile variable, will see a fully constructed object. There is no need for further synchronization. This pattern is usually seen in combination with immutable types.
class Tree {
private volatile Node node;
public void update() {
node = new Node(...);
}
public Node get() {
return node;
}
}
Regarding the first question. You can use volatile variables to synchronize access to non-volatile variable. The following listing shows an example. Imagine that the two variables are initialized as shown, and that the two methods are executed concurrently. It is guaranteed, that if the second thread sees the update to foo, it will also see the update to bar.
volatile int foo = 0;
int bar = 0;
void thread1() {
bar = 1;
foo = 1; // write to volatile variable
}
void thread2() {
if (foo == 1) { // read from volatile variable
int r = bar; // r == 1
}
}
However, your example is different. The reading and writing might look as follows. In contrast to the above example, both threads read from the volatile variable. However, read operations on volatile variables do not synchronize with each other.
void thread1() {
Node temp = root; // read from volatile variable
temp.numOfKeys = 1;
}
void thread2() {
Node temp = root; // read from volatile variable
int r = temp.numOfKeys;
}
In other words: If thread A writes to a volatile variable x and thread B reads the value written to x, then after the read operation, thread B will see all write operations of thread A, that occurred before the write to x. But without a write operation to a volatile variable, there is no effect on updates to other variables.
That sounds more complicated than it actually is. Actually, there is only one rule to consider, which you can find in JLS8 §17.4.5:
[..] If all sequentially consistent executions are free of data races, [..] then all executions of the program will appear to be sequentially consistent.
Simply put, a data race exists if two threads can access the same variable at the same time, at least one operation is a write operation, and the variable is non-volatile. Data races can be eliminated by declaring shared variables as volatile. Without data races, there is no problem with visibility of updates.