Hook线程以及捕获线程执行异常
1、获取线程运行时异常
1.1、在Thread类中，关于处理运行时异常的API总共有四个，如下所示：
public static void setUncaughtExceptionHandler(UncaughtExceptionHandler eh) ：为某个特定线程指定UncaughtExceptionHandler
public static void setDefaultUncaughtExceptionHandler(UncaughtExceptionHandler eh) ：设置全局的UncaughtExceptionHandler
public static void getUncaughtExceptionHandler(UncaughtExceptionHandler eh) ：获取特定线程的UncaughtExceptionHandler
public static void getDefaultUncaughtExceptionHandler(UncaughtExceptionHandler eh)：获取全局的UncaughtExceptionHandler
1.2、UncaughtExceptionHandler的介绍
线程在执行单元中时不允许抛出checked异常的，而且线程运行在自己的上下文中，派生他的线程将无法直接获得它运行中出现的异常信息。对此，
Java为我们提供了一个UncaughtExceptionHandler接口，当线程在运行过程中出现异常时，会回调UncaughtExceptionHandler接口，从而得知时那个线程在
运行时出错，以及出现了什么样的错误，源码如下：
@FunctionalInterface
public interface UncaughtExceptionHandler {
/**
* Method invoked when the given thread terminates due to the
* given uncaught exception.
* <p>Any exception thrown by this method will be ignored by the
* Java Virtual Machine.
* @param t the thread
* @param e the exception
*/
void uncaughtException(Thread t, Throwable e);
}在上述代码中，UncaughtExceptionHandler时一个FunctionInterface，只有一个抽象方法，该回调接口会被Thread中的dispatchUncaughtException方法调用，如下所示：
/**
* 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);
}当线程在运行过程中出现异常时，JVM会调用dispatchUncaughtException方法，该方法会将对应的线程实例以及异常信息传递给回调接口。
1.3 UncaughtExceptionHandler实例
public class CaptureThreadException {
public static void main(String[] args) {
//设置全局异常回调接口
Thread.setDefaultUncaughtExceptionHandler((t,e)->{
System.out.println(t.getName()+" occur exception");
e.printStackTrace();
});
final Thread thread=new Thread(()->{
try {
TimeUnit.SECONDS.sleep(2);
} catch (InterruptedException e) {
}
//here will throw unchecked exception
System.out.println(1/0);
},"Test-Thread");
thread.start();
}
}执行上面的程序，线程Test-Thread在运行两秒之后会抛出一个unchecked异常，我们设置的回调接口将获得该异常信息，程序的执行结果如下：
1.4 UncaughtExceptionHandler源码分析
在没有向线程注入UncaughtExceptionHandler回调接口的情况下，线程若出现了异常又将如何处理呢？下面我们将通过对Thread的源码进行分析来追踪一下，示例代码如下：
/**
* 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;
}getUncaughtExceptionHandler方法首先会判断当前线程是否设置了handler，如果有则执行线程自己的uncaughtException方法，否则就到所在的ThreadGroup中获取，ThreadGroup同样也实现了UncaughtExceptionHandler接口，下面再来看看ThreadGroup的uncaughtException方法：
/**
* 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);
}
}
}（1）该ThreadGroup如果有父ThreadGroup，则直接调用父Group的uncaughtException方法
（2）如果设置了全局默认的UncaughtExcpetionHandler，则调用uncaughtException方法
（3）若既没有父ThreadGroup，也没有设置全局默认的UncaughtExceptionHandler，则会直接将异常的堆栈信息定向到Systerm.err中
下面是没有设置默认的Handler，也没有对thread指定Handler，因此当thread出现异常时，会向上寻找Group的uncaughtException方法
public class EmptyExceptionHandler {
public static void main(String[] args) {
//get current thread's thread group
ThreadGroup mainGroup = Thread.currentThread().getThreadGroup();
System.out.println(mainGroup.getName());
System.out.println(mainGroup.getParent());
System.out.println(mainGroup.getParent().getParent());
final Thread thread=new Thread(()->{
try {
TimeUnit.SECONDS.sleep(2);
} catch (InterruptedException e) {
}
//here will throw unchecked exception
System.out.println(1/0);
},"Test-Thread");
thread.start();
}
}执行结果：