In the code and output below, t2 doesn't start until t1 finishes. Shouldn't they work parallel? Is Thread.sleep() affect whole process?
public class Main {
public static void main(String[] args) {
T t1 = new T(), t2 = new T();
t1.run();
t2.run();
}
}
class Test {
private int x;
void foo() {
synchronized (this){
System.out.println("Entered");
try {
Thread.sleep(5000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("Exit");
}
}
}
class T extends Thread {
static Test t = new Test();
public void run() {
System.out.println("Thread started");
t.foo();
}
}
Output:
Thread started
Entered
Exit
Thread started
Entered
Exit
If you want to run these as separate threads, you need to call the Thread.start() method.
Instead, you're calling the run() method directly. The two calls will execute in the same thread as the caller.
As an aside, usually you can just subclass Runnable rather than Thread. Then you can choose to pass your Runnable to the Thread(Runnable) constructor -- or to an ExecutorService.
Related
I am trying to figure out how Thread.sleep works, so I create the following piece of code:
public static void main() {
Runnable runnable = new Runnable() {
public void run() {
try {
Thread.sleep(1000);
System.out.println("Middle");
} catch (InterruptedException e) {
}
}
};
System.out.println("Before");
Thread t2 = new Thread(runnable);
t2.start();
System.out.println("After");
However, it only prints Before and After in the console and skips the Middle.
So I am wondering whether this Thread.sleep will break the Runnable part??
On my machine output of your code is:
Before
After
Middle
If you want to print Middle before After you need to add t2.join() method call right after t2.start(). Join method explanation.
Use join () to make your main thread wait till your thread (t2) execution finishes.
public static void main(String[] args) throws InterruptedException {
Runnable runnable = new Runnable() {
public void run() {
try {
Thread.sleep(1000);
System.out.println("Middle");
} catch (InterruptedException e) {
}
}
};
System.out.println("Before");
Thread t2 = new Thread(runnable);
t2.start();
t2.join();
System.out.println("After");
Output:
Before
Middle
After
Refer to this post to know about join() : What does this thread join code mean?
Refer to this post to know about the main thread: When does the main thread stop in Java?
I have written below sample code to test behavior of threads . But main is not starting thread test1 execution , Please let me know what is the problem with this code
class test1 implements Runnable
{
Thread t ;
test1(String Name)
{
t = new Thread(Name);
t.start();
}
#Override
public void run() {
// TODO Auto-generated method stub
System.out.println("Entered test1 run");
}
}
public class SampleThread{
public static void main(String[] args) {
Thread r = Thread.currentThread();
System.out.println(r.getName()+" "+r.getPriority());
r.setName("MainThread");
r.setPriority(8);
test1 t1 = new test1("test1");
System.out.println("calling threads");
try
{
t1.t.join();
}
catch(Exception e)
{
e.printStackTrace();
}
}
}
Changing t = new Thread(Name); to t = new Thread(this, Name); (in test1) would make your code work.
But still your code it quite confusing. You can make a java thread by sub-classing Thread or implementing Runnable and passing the runnable to a thread, and you seem to trying to do both at the same time (incorrectly).
I suggest you look up a guide on threads like https://www.tutorialspoint.com/java/java_multithreading.htm
public class Test2 {
static int count;
public static void main(String[] args) {
final Test2 t1 = new Test2();
final Test2 t2 = new Test2();
new Thread(new Runnable() {
#Override
public void run() {
t1.foo();
}
}).start();
new Thread(new Runnable() {
#Override
public void run() {
t1.bar();
}
}).start();
new Thread(new Runnable() {
#Override
public void run() {
t2.foo();
}
}).start();
}
public static synchronized void foo() {
synchronized (Test2.class) {
System.out.println("run bar"+count++);
try {
Thread.sleep(100000);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
public static synchronized void bar() {
System.out.println("run bar");
try {
Thread.sleep(10000);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
above are codes which I tried.When I write all codes in one Class synchronized (Test2.class) and I found something strange happened.After I invoked foo() method I can't invoke bar() method immediately. what I think it lock same object.how to explain this strange thing.
All 3 of the threads your code starts acquire the same lock, on the Test2 class. When you write a method starting with
static synchronized
that means it has to acquire the lock on the class object.
The synchronized block in the foo method is redundant. It specifies using the same class to lock on as using static synchronized. Since intrinsic locks are reentrant this doesn't cause a problem.
Anyway, each of your threads acquires the lock, runs to completion, then releases the lock. Since you are locking on the class and not on an instance it doesn't matter which instance of Test2 your threads use, they still acquire the same lock and execute one at a time.
When I ran this I got the following output:
c:\Users\ndh>java Test2
run bar0
run bar1
run bar
The order in which these threads run is up to the scheduler. It would seem reasonable to guess that the Runnable calling t1.foo got a head start -- since it was created and started first it's likely to have a window where it won't have any competition acquiring the lock.
public class DeadLock {
public static void main(String[] args) {
final A a = new A();
final B b = new B();
new Thread(new Runnable(){
#Override
public void run() {
a.aMethod(b);
}
},"Thread-2").start();
new Thread(new Runnable(){
#Override
public void run() {
b.bMethod(a);
}
},"Thread-2").start();
}
}
class A {
public void aMethod(B b) {
System.out.println("A method");
}
}
class B {
public void bMethod(A a) {
System.out.println("B method");
}
}
I understand that Deadlock occurs when two or more threads are blocked waiting for each other. How do I implement the same using the code above? Synchronizing the methods in classes A and B doesn't help.
How do I implement the same using the code above? Synchronizing the methods in classes A and B doesn't help.
The definition of deadlock is that A is locked and needs the lock from B at the same time that B is locked and needs the lock from A.
You aren't going to be able to simulate it with a single thread call because likely the first thread that is started will finish before the second thread starts. This is a race condition where the threads are racing to deadlock or not.
You need to loop in both threads and try the dual lock over and over. Something like the following should work. At some point you will see the output stop.
public void run() {
while (true) {
a.aMethod(b);
}
}
...
public void run() {
while (true) {
b.bMethod(a);
}
}
...
public synchronized void aMethod(B b) {
System.out.println("B method");
b.bMethod(this);
}
...
public synchronized void aMethod(A a) {
System.out.println("A method");
a.aMethod(this);
}
You may also have to remove the System.out.println(...) calls because they also are synchronized which will change the timing of your program and may make it harder to hit a deadlock. Without the output, to detect the deadlock without the output you can attach to the process using jconsole, look at the Threads tab, and click "Detect Deadlock". You can also watch the load of your program. It should be ~200% while 2 threads are spinning and then go to 0 when they are deadlocked.
public static void main(String[] args) {
final Object a = new Object();
final Object b = new Object();
Thread t1 = new Thread() {
#Override
public void run() {
synchronized (a) {
try {
sleep(10000);
} catch (InterruptedException exc) {
//
}
synchronized (b) {
//
}
}
}
};
Thread t2 = new Thread() {
#Override
public void run() {
synchronized (b) {
try {
sleep(10000);
} catch (InterruptedException exc) {
//
}
synchronized (a) {
//
}
}
}
};
t1.start();
t2.start();
}
I was reading this post and the suggestions given to interrupt one thread from another is
" " " Here are a couple of approaches that should work, if implemented correctly.
You could have both threads regularly check some common flag variable (e.g. call it stopNow), and arrange that both threads set it when they finish. (The flag variable needs to be volatile ... or properly synchronized.)
You could have both threads regularly call the Thread.isInterrupted() method to see if it has been interrupted. Then each thread needs to call Thread.interrupt() on the other one when it finishes." " "
I do not understand how the second approach is possible that is using Thread.isInterrupted().
That is, how can Thread-1 call Thread.interrupt() on Thread-2.
Consider this example, in the main method I start two threads t1 and t2. I want t1 to stop t2 after reaching certain condition. how can I achieve this?
class Thread1 extends Thread {
public void run(){
while (!isDone){
// do something
}
} //now interrupt Thread-2
}
class Thread2 extends Thread {
public void run(){
try {
while(!Thread.isInterupted()){
//do something;
}
catch (InterruptedExecption e){
//do something
}
}
}
public class test {
public static void main(String[] args){
try {
Thread1 t1 = new Thread1();
Thread2 t2 = new Thread2();
t1.start();
t2.start();
} catch (IOException e) {
e.printStackTrace();
}
}
}
The context of this is that you are trying to implement your scheme using thread interrupts.
In order for that to happen, the t1 object needs the reference to the t2 thread object, and then it simply calls t2.interrupt().
There are a variety of ways that t1 could get the reference to t2.
It could be passed as a constructor parameter. (You would need to instantiate Thread2 before Thread1 ...)
It could be set by calling a setter on Thread1.
It could be retrieved from a static variable or array, or a singleton "registry" object of some kind.
It could be found by enumerating all of the threads in the ThreadGroup looking for one that matches t2's name.
public class test {
private static boolean someCondition = true;
public static void main(String[]args){
Thread t2 = new Thread(new someOtherClass("Hello World"));
Thread t1 = new Thread(new someClass(t2));
t2.start();
t1.start();
try {
t1.join();
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
static class someClass implements Runnable{
Thread stop;
public someClass(Thread toStop){
stop = toStop;
}
public void run(){
while(true){
try {
Thread.sleep(500);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
if(someCondition && !stop.isInterrupted()){
stop.interrupt();
}
}
}
}
static class someOtherClass implements Runnable{
String messageToPrint;
public someOtherClass(String s){
messageToPrint = s;
}
public void run(){
while(true){
try {
Thread.sleep(500);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
System.out.println(messageToPrint);
}
}
}
}
You could consider the use of Future interface. It provides a cancel() method.
http://docs.oracle.com/javase/7/docs/api/java/util/concurrent/Future.html
Playing with interruption makes your life unnecessarily hard. Besides the fact that your code must know the threads, interruption does not provide any context information about the reason of the interruption.
If you have a condition that is shared by your code possibly executed by different threads, just encapsulate that condition into an object and share that object:
public class Test {
public static void main(String[] args) {
Condition c=new Condition();
new Thread(new Setter(c)).start();
new Thread(new Getter(c, "getter 1")).start();
// you can simply extend it to more than one getter:
new Thread(new Getter(c, "getter 2")).start();
}
}
class Getter implements Runnable {
final Condition condition;
final String name;
Getter(Condition c, String n) { condition=c; name=n; }
public void run() {
while(!condition.isSatisfied()) {
System.out.println(name+" doing something else");
try { Thread.sleep(300); } catch(InterruptedException ex){}
}
System.out.println(name+" exiting");
}
}
class Setter implements Runnable {
final Condition condition;
Setter(Condition c) { condition=c; }
public void run() {
System.out.println("setter: doing my work");
try { Thread.sleep(3000); }
catch(InterruptedException ex){}
System.out.println("setting condition to satisfied");
condition.setSatisfied();
}
}
class Condition {
private volatile boolean satisfied;
public void setSatisfied() {
satisfied=true;
}
public boolean isSatisfied() {
return satisfied;
}
}
The big advantage of this encapsulation is that it is easy to extend. Suppose you want to allow a thread to wait for the condition instead of polling it. Taking the code above it’s easy:
class WaitableCondition extends Condition {
public synchronized boolean await() {
try {
while(!super.isSatisfied()) wait();
return true;
} catch(InterruptedException ex){ return false; }
}
public synchronized void setSatisfied() {
if(!isSatisfied()) {
super.setSatisfied();
notifyAll();
}
}
}
class Waiter implements Runnable {
final WaitableCondition condition;
final String name;
Waiter(WaitableCondition c, String n) { condition=c; name=n; }
public void run() {
System.out.println(name+": waiting for condition");
boolean b=condition.await();
System.out.println(name+": "+(b? "condition satisfied": "interrupted"));
}
}
Without changing the other classes you can now extend your test case:
public class Test {
public static void main(String[] args) {
WaitableCondition c=new WaitableCondition();
new Thread(new Setter(c)).start();
new Thread(new Getter(c, "getter 1")).start();
// you can simply extend it to more than one getter:
new Thread(new Getter(c, "getter 2")).start();
// and you can have waiters
new Thread(new Waiter(c, "waiter 1")).start();
new Thread(new Waiter(c, "waiter 2")).start();
}
}