线程未捕获的异常的处理

多线程 Java
  1. 线程未捕获异常处理
  2. 未捕获异常处理流程代码分析

线程未捕获异常处理

线程中run方法出现异常会导致线程终止运行，可以在run方法中使用try-catch捕获发生的异常，对于run方法中没有用try-catch捕获到的异常，如果想要对其进行处理，需要给线程设置异常捕获处理器。

线程中抛出的异常并不能被如下形式的try-catch捕获到：

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try{
    new Thread(new Runnable() {
        @Override
        public void run() {
            int i=1/0;
        }
    }).start();
}catch (Exception e){
    System.out.println("error:"+e.getMessage());
}

Thread中的运行时异常并不能被catch捕获，System.out.println(“error:”+e.getMessage())并没有执行。

如果我们想要对Thread中抛出的未被捕获的异常进行处理，我们可以给线程设置异常捕获处理器。

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Thread thread =new Thread(new Runnable() {
     @Override
     public void run() {
        int i=1/0;
     }
});
thread.setUncaughtExceptionHandler(new Thread.UncaughtExceptionHandler() {
    @Override
    public void uncaughtException(Thread t, Throwable e) {
        System.out.println("error:"+e.getMessage());
    }
 });
thread.start();

线程中抛出的异常会被setUncaughtExceptionHandler方法设置的异常捕获处理器捕获， System.out.println(“error:”+e.getMessage())获得执行。

我们可以给每个线程的设置具体的未捕获异常处理器，也可以给所有的线程设置默认异常处理器。通过

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Thread.setDefaultUncaughtExceptionHandler(new Thread.UncaughtExceptionHandler() {
    @Override
    public void uncaughtException(Thread t, Throwable e) {
                    
    }
});

可以给所有线程设置默认的处理器。给所有的线程设置了默认的处理器后，对于没有设置特定的处理器的线程，当线程中出现未捕获的异常的时候，会使用默认的处理器来处理线程中出现的未捕获的异常。

如果没有给线程设置特定的处理器，那么线程的处理器就是线程的ThreadGroup对象，我们通过Thread.setDefaultUncaughtExceptionHandler设置的异常处理器的uncaughtException方法会在ThreadGroup对象的uncaughtException方法中被调用，具体看下一小节源码分析。

未捕获异常处理流程代码分析

当线程中出现未捕获异常后JVM将调用Thread中的dispatchUncaughtException方法把异常传递给线程的未捕获异常处理器。

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/**
* Dispatch an uncaught exception to the handler. This method is
* intended to be called only by the JVM.
*/
private void dispatchUncaughtException(Throwable e) {
    getUncaughtExceptionHandler().uncaughtException(this, e);
}

dispatchUncaughtException这个方法只是提供给JVM调用的。

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/* Returns the handler invoked when this thread abruptly terminates
    * due to an uncaught exception. If this thread has not had an
    * uncaught exception handler explicitly set then this thread's
    * <tt>ThreadGroup</tt> object is returned, unless this thread
    * has terminated, in which case <tt>null</tt> is returned.
    * @since 1.5
    * @return the uncaught exception handler for this thread
    */
   public UncaughtExceptionHandler getUncaughtExceptionHandler() {
       return uncaughtExceptionHandler != null ?uncaughtExceptionHandler : group;
   }

从代码可以看出**如果没有设置uncaughtExceptionHandler，将使用线程所在的线程组来处理这个未捕获异常。**线程组ThreadGroup实现了UncaughtExceptionHandler，所以可以用来处理未捕获异常。ThreadGroup类定义：

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class ThreadGroup implements Thread.UncaughtExceptionHandler

ThreadGroup实现的uncaughtException如下：

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/**
 * Called by the Java Virtual Machine when a thread in this
 * thread group stops because of an uncaught exception, and the thread
 * does not have a specific {@link Thread.UncaughtExceptionHandler}
 * installed.
 * <p>
 * The <code>uncaughtException</code> method of
 * <code>ThreadGroup</code> does the following:
 * <ul>
 * <li>If this thread group has a parent thread group, the
 *     <code>uncaughtException</code> method of that parent is called
 *     with the same two arguments.
 * <li>Otherwise, this method checks to see if there is a
 *     {@linkplain Thread#getDefaultUncaughtExceptionHandler default
 *     uncaught exception handler} installed, and if so, its
 *     <code>uncaughtException</code> method is called with the same
 *     two arguments.
 * <li>Otherwise, this method determines if the <code>Throwable</code>
 *     argument is an instance of {@link ThreadDeath}. If so, nothing
 *     special is done. Otherwise, a message containing the
 *     thread's name, as returned from the thread's {@link
 *     Thread#getName getName} method, and a stack backtrace,
 *     using the <code>Throwable</code>'s {@link
 *     Throwable#printStackTrace printStackTrace} method, is
 *     printed to the {@linkplain System#err standard error stream}.
 * </ul>
 * <p>
 * Applications can override this method in subclasses of
 * <code>ThreadGroup</code> to provide alternative handling of
 * uncaught exceptions.
 *
 * @param   t   the thread that is about to exit.
 * @param   e   the uncaught exception.
 * @since   JDK1.0
 */
public void uncaughtException(Thread t, Throwable e) {
    if (parent != null) {
        parent.uncaughtException(t, e);
    } else {
        Thread.UncaughtExceptionHandler ueh =
            Thread.getDefaultUncaughtExceptionHandler();
        if (ueh != null) {
            ueh.uncaughtException(t, e);
        } else if (!(e instanceof ThreadDeath)) {
            System.err.print("Exception in thread \""
                             + t.getName() + "\" ");
            e.printStackTrace(System.err);
        }
    }
}

线程组处理未捕获异常，首先将异常消息通知给父线程组；如果父线程组为空，尝试利用一个默认的defaultUncaughtExceptionHandler获得异常处理器来处理异常，Thread.getDefaultUncaughtExceptionHandler()获得的异常捕获处理器就是我们之前通过Thread.getDefaultUncaughtExceptionHandler(）设置的处理器；如果没有默认的异常处理器则将错误信息输出到System.err。

如果不给线程设置线程组，仍然可以正常的使用默认异常处理器。所有的线程都会有线程组，即使我们在构造线程时显示的传入null值的线程组，最终JVM也会为我们分配到一个线程组。Thread的init方法决定了这个特性：

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if (g == null) {
        /* Determine if it's an applet or not */
        /* If there is a security manager, ask the security manager
           what to do. */
        if (security != null) {
            g = security.getThreadGroup();
        }
        /* If the security doesn't have a strong opinion of the matter
           use the parent thread group. */
        if (g == null) {
            g = parent.getThreadGroup();
        }
}

##几个小例子

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Thread.setDefaultUncaughtExceptionHandler(new Thread.UncaughtExceptionHandler() {
    @Override
    public void uncaughtException(Thread t, Throwable e) {
            System.out.println("I catch a exception from  " + Thread.currentThread().getName() + ":" + Thread.currentThread().getThreadGroup().getName());
        }
    });
    
    ThreadGroup myGroup = new ThredGroup("ThreadGroup");
    
    new Thread(myGroup, new Runnable() {
        @Override
        public void run() {
            int i = 1/0;
        }
    }, "thread1").start();

    new Thread(myGroup, new Runnable() {
        @Override
        public void run() {
            int i = 1/0;
        }
    }, "thread2").start();

打印结果如下：

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I catch a exception from  thread1:myGroup
I catch a exception from  thread2:myGroup

通过自定义ThreadGrop改变上述机制。

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class OtherGroup extends ThreadGroup{
    public OtherGroup(String name) {
        super(name);
    }

    @Override
    public void uncaughtException(Thread t, Throwable e) {
        System.out.println("I am a other group and do nothing");
    }
}

 Thread.setDefaultUncaughtExceptionHandler(new Thread.UncaughtExceptionHandler() {
    @Override
    public void uncaughtException(Thread t, Throwable e) {
            System.out.println("I catch a exception from  " +  Thread.currentThread().getName() + ":" + Thread.currentThread().getThreadGroup().getName());
        }
    });
    
    ThreadGroup myGroup = new OtherGroup("OtherGroup");
    
    new Thread(myGroup, new Runnable() {
        @Override
        public void run() {
            int i = 1/0;
        }
    }, "thread1").start();

    new Thread(myGroup, new Runnable() {
        @Override
        public void run() {
            int i = 1/0;
        }
    }, "thread2").start();

打印结果如下：

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I am a other group and do nothing
I am a other group and do nothing

如果在OtherGroup的uncaughtException加上如下代码：

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super.uncaughtException(t,e);

那么执行结果会是：

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I catch a exception from  thread1:badGroup
I am a bad group and do nothing
I catch a exception from  thread2:badGroup
I am a bad group and do nothing

参考文献：
Java核心技术讲义
Java线程未捕获异常处理
《Java并发》：第五节 线程异常处理
Java多线程——<七>多线程的异常捕捉

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