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安全完美終止線程大全，各種各樣情況 轉載自： http://www.blogjava.net/jinfeng_wang/archive/2008/04/27/196477.html 程序是很簡易的。然而，在編程人員面前，多線程呈現出了一組新的難題，如果沒有被恰當的解決，將導致意外的行爲以及細微的、難以發現的錯誤。 在本篇文章中，我們針對這些難題之一：如何中斷一個正在運行的線程。背景 中斷（Interrupt）一個線程意味着在該線程完成任務之前停止其正在進行的一切，有效地中止其當前的操作。線程是死亡、還是等待新的任務或是繼續運行至下一步，就取決於這個程序。雖然初次看來它可能顯得簡單，但是，你必須進行一些預警以實現期望的結果。你最好還是牢記以下的幾點告誡。 首先，忘掉Thread.stop方法。雖然它確實停止了一個正在運行的線程，然而，這種方法是不安全也是不受提倡的，這意味着，在未來的JAVA版本中，它將不復存在。 |
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一些輕率的傢伙可能被另一種方法Thread.interrupt所迷惑。儘管，其名稱似乎在暗示着什麼，然而，這種方法並不會中斷一個正在運行的線程（待會將進一步說明），正如Listing A中描述的那樣。它創建了一個線程，並且試圖使用Thread.interrupt方法停止該線程。Thread.sleep()方法的調用，爲線程的初始化和中止提供了充裕的時間。線程本身並不參與任何有用的操作。 class Example1 extends Thread {
boolean stop=false;
public static void main( String args[] ) throws Exception {
Example1 thread = new Example1();
System.out.println( "Starting thread..." );
thread.start();
Thread.sleep( 3000 );
System.out.println( "Interrupting thread..." );
thread.interrupt();
Thread.sleep( 3000 );
System.out.println("Stopping application..." );
//System.exit(0);
}
public void run() {
while(!stop){
System.out.println( "Thread is running..." );
long time = System.currentTimeMillis();
while((System.currentTimeMillis()-time < 1000)) {
}
}
System.out.println("Thread exiting under request..." );
}
}
如果你運行了Listing A中的代碼，你將在控制檯看到以下輸出： |
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Writing multithreaded programs in Java, with its built-in support for threads, is fairly straightforward. However, multithreading presents a whole set of new challenges to the programmer that, if not correctly addressed, can lead to unexpected behavior and subtle, hard-to-find errors. In this article, we address one of those challenges: how to interrupt a running thread.
Background
Interrupting a thread means stopping what it is doing before it has completed its task, effectively aborting its current operation. Whether the thread dies, waits for new tasks, or goes on to the next step depends on the application.
Although it may seem simple at first, you must take some precautions in order to achieve the desired result. There are some caveats you must be aware of as well.
First of all, forget the Thread.stop method. Although it indeed stops a running thread, the method is unsafe and was deprecated, which means it may not be available in future versions of the Java.
Another method that can be confusing for the unadvised is Thread.interrupt. Despite what its name may imply, the method does not interrupt a running thread (more on this later), as Listing A demonstrates. It creates a thread and tries to stop it using Thread.interrupt. The calls toThread.sleep() give plenty of time for the thread initialization and termination. The thread itself does not do anything useful.
If you run the code in Listing A, you should see something like this on your console:
Starting thread...
Thread is running...
Thread is running...
Thread is running...
Interrupting thread...
Thread is running...
Thread is running...
Thread is running...
Stopping application...
Even after Thread.interrupt() is called, the thread continues to run for a while.
Really interrupting a thread
The best, recommended way to interrupt a thread is to use a shared variable to signal that it must stop what it is doing. The thread must check the variable periodically, especially during lengthy operations, and terminate its task in an orderly manner. Listing B demonstrates this technique.
Running the code in Listing B will generate output like this (notice how the thread exits in an orderly fashion):
Starting thread...
Thread is running...
Thread is running...
Thread is running...
Asking thread to stop...
Thread exiting under request...
Stopping application...
Although this method requires some coding, it is not difficult to implement and give the thread the opportunity to do any cleanup needed, which is an absolute requirement for any multithreaded application. Just be sure to declare the shared variable as volatile or enclose any access to it intosynchronized blocks/methods.
So far, so good! But what happens if the thread is blocked waiting for some event? Of course, if the thread is blocked, it can't check the shared variable and can't stop. There are plenty of situations when that may occur, such as calling Object.wait(), ServerSocket.accept(), andDatagramSocket.receive(), to name a few.
They all can block the thread forever. Even if a timeout is employed, it may not be feasible or desirable to wait until the timeout expires, so a mechanism to prematurely exit the blocked state must be used.
Unfortunately there is no such mechanism that works for all cases, but the particular technique to use depends on each situation. In the following sections, I'll give solutions for the most common cases.
Interrupting a thread with Thread.interrupt()
As demonstrated in Listing A, the method Thread.interrupt() does not interrupt a running thread. What the method actually does is to throw an interrupt if the thread is blocked, so that it exits the blocked state. More precisely, if the thread is blocked at one of the methods Object.wait,Thread.join, or Thread.sleep, it receives an InterruptedException, thus terminating the blocking method prematurely.
So, if a thread blocks in one of the aforementioned methods, the correct way to stop it is to set the shared variable and then call the interrupt() method on it (notice that it is important to set the variable first). If the thread is not blocked, calling interrupt() will not hurt; otherwise, the thread will get an exception (the thread must be prepared to handle this condition) and escape the blocked state. In either case, eventually the thread will test the shared variable and stop. Listing C is a simple example that demonstrates this technique.
As soon as Thread.interrupt() is called in Listing C, the thread gets an exception so that it escapes the blocked state and determines that it should stop. Running this code produces output like this:
Starting thread...
Thread running...
Thread running...
Thread running...
Asking thread to stop...
Thread interrupted...
Thread exiting under request...
Stopping application...
Interrupting an I/O operation
But what happens if the thread is blocked on an I/O operation? I/O can block a thread for a considerable amount of time, particularly if network communication is involved. For example, a server may be waiting for a request, or a network application may be waiting for an answer from a remote host.
If you're using channels, available with the new I/O API introduced in Java 1.4, the blocked thread will get a ClosedByInterruptException exception. If that is the case, the logic is the same as that used in the third example—only the exception is different.
But you might be using the traditional I/O available since Java 1.0, since the new I/O is so recent and requires more work. In this case, Thread.interrupt() doesn't help, since the thread will not exit the blocked state. Listing D demonstrates that behavior. Although the interrupt() method is called, the thread does not exit the blocked state.
Fortunately, the Java Platform provides a solution for that case by calling the close() method of the socket the thread is blocked in. In this case, if the thread is blocked in an I/O operation, the thread will get a SocketException exception, much like the interrupt() method causes anInterruptedException to be thrown.
The only caveat is that a reference to the socket must be available so that its close() method can be called. That means the socket object must also be shared. Listing E demonstrates this case. The logic is the same as in the examples presented so far.
And here's the sample output you can expect from running Listing E:
Starting thread...
Waiting for connection...
Asking thread to stop...
accept() failed or interrupted...
Thread exiting under request...
Stopping application...
Multithreading is a powerful tool, but it presents its own set of challenges. One of these is how to interrupt a running thread. If properly implemented, these techniques make interrupting a thread no more difficult than using the built-in operations already provided by the Java Platform.
