欢迎您访问程序员文章站本站旨在为大家提供分享程序员计算机编程知识!
您现在的位置是: 首页  >  IT编程

JAVA多线程之中断机制stop()、interrupted()、isInterrupted()

程序员文章站 2024-03-11 19:30:13
一,介绍 本文记录java多线程中的中断机制的一些知识点。主要是stop方法、interrupted()与isinterrupted()方法的区别,并从源代码的实现上进行...

一,介绍

本文记录java多线程中的中断机制的一些知识点。主要是stop方法、interrupted()与isinterrupted()方法的区别,并从源代码的实现上进行简单分析。

java中有3种方式可以终止正在运行的线程

①线程正常退出,即run()方法执行完毕了

②使用thread类中的stop()方法强行终止线程。但stop()方法已经过期了,不推荐使用

③使用中断机制

线程正常退出没有什么东东,中断机制下面详细介绍,先看下stop()方法的源代码,关键是源代码上的注释。它解释了为什么stop()不安全,stop()方法停止的是哪个线程?

/**
* forces the thread to stop executing.
* <p>
* if there is a security manager installed, its <code>checkaccess</code>
* method is called with <code>this</code>
* as its argument. this may result in a
* <code>securityexception</code> being raised (in the current thread).
* <p>
* if this thread is different from the current thread (that is, the current
* thread is trying to stop a thread other than itself), the
* security manager's <code>checkpermission</code> method (with a
* <code>runtimepermission("stopthread")</code> argument) is called in
* addition.
* again, this may result in throwing a
* <code>securityexception</code> (in the current thread).
* <p>
* the thread represented by this thread is forced to stop whatever
* it is doing abnormally and to throw a newly created
* <code>threaddeath</code> object as an exception.
* <p>
* it is permitted to stop a thread that has not yet been started.
* if the thread is eventually started, it immediately terminates.
* <p>
* an application should not normally try to catch
* <code>threaddeath</code> unless it must do some extraordinary
* cleanup operation (note that the throwing of
* <code>threaddeath</code> causes <code>finally</code> clauses of
* <code>try</code> statements to be executed before the thread
* officially dies). if a <code>catch</code> clause catches a
* <code>threaddeath</code> object, it is important to rethrow the
* object so that the thread actually dies.
* <p>
* the top-level error handler that reacts to otherwise uncaught
* exceptions does not print out a message or otherwise notify the
* application if the uncaught exception is an instance of
* <code>threaddeath</code>.
*
* @exception securityexception if the current thread cannot
* modify this thread.
* @see #interrupt()
* @see #checkaccess()
* @see #run()
* @see #start()
* @see threaddeath
* @see threadgroup#uncaughtexception(thread,throwable)
* @see securitymanager#checkaccess(thread)
* @see securitymanager#checkpermission
* @deprecated this method is inherently unsafe. stopping a thread with
* thread.stop causes it to unlock all of the monitors that it
* has locked (as a natural consequence of the unchecked
* <code>threaddeath</code> exception propagating up the stack). if
* any of the objects previously protected by these monitors were in
* an inconsistent state, the damaged objects become visible to
* other threads, potentially resulting in arbitrary behavior. many
* uses of <code>stop</code> should be replaced by code that simply
* modifies some variable to indicate that the target thread should
* stop running. the target thread should check this variable
* regularly, and return from its run method in an orderly fashion
* if the variable indicates that it is to stop running. if the
* target thread waits for long periods (on a condition variable,
* for example), the <code>interrupt</code> method should be used to
* interrupt the wait.
* for more information, see
* <a href="{@docroot}/../technotes/guides/concurrency/threadprimitivedeprecation.html">why
* are thread.stop, thread.suspend and thread.resume deprecated?</a>.
*/
@deprecated
public final void stop() {
stop(new threaddeath());
}

上面注释,第9行到第16行表明,stop()方法可以停止“其他线程”。执行thread.stop()方法这条语句的线程称为当前线程,而“其他线程”则是 调用thread.stop()方法的对象thread所代表的线程。

如:

public static void main(string[] args) {
mythread thread = new mythread...
//.....
thread.stop();
//....
}

在main方法中,当前线程就是main线程。它执行到第4行,想把“其他线程”thread“ 给停止。这个其他线程就是mythread类 new 的thread对象所表示的线程。

第21行至23行表明,可以停止一个尚未started(启动)的线程。它的效果是:当该线程启动后,就立马结束了。

第48行以后的注释,则深刻表明了为什么stop()方法被弃用!为什么它是不安全的。

比如说,threada线程拥有了监视器,这些监视器负责保护某些临界资源,比如说银行的转账的金额。当正在转账过程中,main线程调用 threada.stop()方法。结果导致监视器被释放,其保护的资源(转账金额)很可能出现不一致性。比如,a账户减少了100,而b账户却没有增加100

二,中断机制

java中如何正确地使用中断机制的细节太多了。interrupted()方法与 isinterrupted()方法都是反映当前线程的是否处于中断状态的。

①interrupted()

/**
* tests whether the current thread has been interrupted. the
* <i>interrupted status</i> of the thread is cleared by this method. in
* other words, if this method were to be called twice in succession, the
* second call would return false (unless the current thread were
* interrupted again, after the first call had cleared its interrupted
* status and before the second call had examined it).
*
* <p>a thread interruption ignored because a thread was not alive
* at the time of the interrupt will be reflected by this method
* returning false.
*
* @return <code>true</code> if the current thread has been interrupted;
* <code>false</code> otherwise.
* @see #isinterrupted()
* @revised .
*/
public static boolean interrupted() {
return currentthread().isinterrupted(true);
}

从源码的注释中看出,它测试的是当前线程(current thread)的中断状态,且这个方法会清除中断状态。

②isinterrupted()

/**
* tests whether this thread has been interrupted. the <i>interrupted
* status</i> of the thread is unaffected by this method.
*
* <p>a thread interruption ignored because a thread was not alive
* at the time of the interrupt will be reflected by this method
* returning false.
*
* @return <code>true</code> if this thread has been interrupted;
* <code>false</code> otherwise.
* @see #interrupted()
* @revised .
*/
public boolean isinterrupted() {
return isinterrupted(false);
}

从源码注释中可以看出,isinterrupted()方法不会清除中断状态。

③interrupted()方法与 isinterrupted()方法的区别

从源代码可以看出,这两个方法都是调用的isinterrupted(boolean clearinterrupted),只不过一个带的参数是true,另一个带的参数是false。

/**
* tests if some thread has been interrupted. the interrupted state
* is reset or not based on the value of clearinterrupted that is
* passed.
*/
private native boolean isinterrupted(boolean clearinterrupted);

因此,第一个区别就是,一个会清除中断标识位,另一个不会清除中断标识位。

再分析源码,就可以看出第二个区别在return 语句上:

public static boolean interrupted() {
return currentthread().isinterrupted(true);
}
/************************/
public boolean isinterrupted() {
return isinterrupted(false);
}

interrupted()测试的是当前的线程的中断状态。而isinterrupted()测试的是调用该方法的对象所表示的线程。一个是静态方法(它测试的是当前线程的中断状态),一个是实例方法(它测试的是实例对象所表示的线程的中断状态)。

下面用个具体的例子来更进一步地阐明这个区别。

有一个自定义的线程类如下:

public class mythread extends thread {
@override
public void run() {
super.run();
for (int i = ; i < ; i++) {
system.out.println("i=" + (i + ));
}
}
}

先看interrupted()方法的示例:

public class run {
public static void main(string[] args) {
try {
mythread thread = new mythread();
thread.start();
thread.sleep();
thread.interrupt();
//thread.currentthread().interrupt();
system.out.println("是否停止?="+thread.interrupted());//false
system.out.println("是否停止?="+thread.interrupted());//false main线程没有被中断!!!
      //......

第5行启动thread线程,第6行使main线程睡眠1秒钟从而使得thread线程有机会获得cpu执行。

main线程睡眠1s钟后,恢复执行到第7行,请求中断 thread线程。

第9行测试线程是否处于中断状态,这里测试的是哪个线程呢???答案是main线程。因为:

(1)interrupted()测试的是当前的线程的中断状态

(2)main线程执行了第9行语句,故main线程是当前线程

再看isinterrupted()方法的示例:

public class run {
public static void main(string[] args) {
try {
mythread thread = new mythread();
thread.start();
thread.sleep();
thread.interrupt();
system.out.println("是否停止?="+thread.isinterrupted());//true

在第8行,是thread对象调用的isinterrupted()方法。因此,测试的是thread对象所代表的线程的中断状态。由于在第7行,main线程请求中断 thread线程,故在第8行的结果为: true