什么时候调用 java 的 thread.run()而不是 thread.start() ？

Call thread.start(), it will in turn call thread.run(). Can't think of a case when you would want to bypass thread.start() and go directly to thread.run()

Never. Calling run() directly just executes the code synchronously (in the same thread), just like a normal method call.

When you want it to run synchronously. Calling the run method won't actually give you multi-threading. The start method creates a new thread which calls the run method.

Taken form the Code Style Java threads FAQ:

Q: What's the difference between a thread's start() and run() methods?

A: The separate start() and run() methods in the Thread class provide two ways to create threaded programs. The start() method starts the execution of the new thread and calls the run() method. The start() method returns immediately and the new thread normally continues until the run() method returns.

The Thread class' run() method does nothing, so sub-classes should override the method with code to execute in the second thread. If a Thread is instantiated with a Runnable argument, the thread's run() method executes the run() method of the Runnable object in the new thread instead.

Depending on the nature of your threaded program, calling the Thread run() method directly can give the same output as calling via the start() method, but in the latter case the code is actually executed in a new thread.

If you want to execute the contents of run() like you would of any other method. Not to start a thread, of course.

You might want to call run() in a particular unit test that is concerned strictly with functionality and not with concurrency.

Assuming that you know the start and run method usage i.e. synchronous vs. asynchronous; run method can be used just to test the functionality.

Plus in some circumstances, the same thread class can be used in two different places with synch and asynch functionality requirements by having two different objects with one's run method and other's start method being invoked.

At least in the JVM 1.6., there's a bit of checking and run is called natively:

 public synchronized void start() {
/**
* This method is not invoked for the main method thread or "system"
* group threads created/set up by the VM. Any new functionality added
* to this method in the future may have to also be added to the VM.
*
* A zero status value corresponds to state "NEW".
*/
if (threadStatus != 0)
throw new IllegalThreadStateException();
group.add(this);
start0();
if (stopBeforeStart) {
stop0(throwableFromStop);
}
}


private native void start0();

This has already been alluded to, but just to be clear: creating a new Thread object only to call it's run() method is needlessly expensive and should be a major red flag. It would be a much better, more decoupled design to create a Runnable impl and either (a) call it's run() method directly if that's the desired behavior, or (b) construct a new Thread with that Runnable and start the Thread.

Better yet, for even more decoupling, check out the Executor interface and framework in JDK 5 and newer. This allows you, in a nutshell, to decouple task execution (the Runnable instance) from how it is executed (the Executor implementation, which might execute the Runnable in the current Thread, in a new Thread, using an existing Thread from a pool, and whatnot).

The separate start() and run() methods in the Thread class provide two ways to create threaded programs. The start() method starts the execution of the new thread and calls the run() method. The start() method returns immediately and the new thread normally continues until the run() method returns.

The Thread class’ run() method does nothing, so sub-classes should override the method with code to execute in the second thread. If a Thread is instantiated with a Runnable argument, the thread’s run() method executes the run() method of the Runnable object in the new thread instead.

Depending on the nature of your threaded program, calling the Thread run() method directly can give the same output as calling via the start() method, but in the latter case the code is actually executed in a new thread.

reference

If the Question was - "why the thread start method is called instead of run method directly" then i have answered with an example code below. Hope that clarifies. In the Example below:

/*
By calling t1.start(),
we are getting the main calling thread returned immediately
after the t1.start() called and is ready to proceed for other
operations.And the thread t1 starts executing the run method of the object r.
Hence the the output will be:


I am the main thread , i created thread t1 and had it execute run method, which is currently looping from 0 to 1000000


I am done executing run method of testThread


*/




/* If we call t1.run() instead of t1.start(), (just replace t1.start() with t1.run() in the code for testing)
its like a regular method call and the main thread will not return until the run method completes,
hence the output will be:


I am done executing run method of testThread


I am the main thread , i created thread t1 and had it execute run method, which is currently looping for i to 1000000


*/




class testThread implements Runnable{


public void run()
{
for(int i=0;i<1000000;i++){} //a simple delay block to clarify.


System.out.println("I am done executing run method of testThread");


}
}


public class mainClass{


public static void main(String [] args)
{
testThread r = new testThread();
Thread t1 = new Thread(r);
t1.start();  /* Question is: can we call instead t1.run() */
System.out.println("I am the main thread , i created thread t1 and had it execute run method, which is currently looping for i to 1000000");


}
}

Executing thread.run() doesn't create a new Thread in which your code gets executed. It just executes the code in the current Thread from which the thread.run() code is invoked.

Executing thread.start() creates a new OS level thread wherein the run() method gets called.

In essence:

Single Threaded programming → Directly calling the run() method

Multi Threaded programming → Calling the start() method

Moreover, as other's have mentioned, 'testing' seems to be the only advisable case wherein you may invoke run() directly from your code.

Just a note to the above great comments: sometimes your write a multi-thread code which uses "start" method to run different threads. You will find it much easier if you use "run" (instead of "start) for debugging since it makes the code to run synchronously and debugging it much easier.

public class TestClass implements Runnable {
public static void main(String[] args) {
TestClass tc = new TestClass();


Thread t1 = new Thread(tc);
System.out.println("Before Starting Thread " + Thread.currentThread().hashCode());
t1.start();
System.out.println("After Starting Thread " + Thread.currentThread().hashCode());
}


@Override
public void run() {
System.out.println("TestClass Run method is  Running with thread " + Thread.currentThread().hashCode());
}
}