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昆明达内培训的老师这一期给大家讲昆明达内培训JAVA Thread线程异常监控

一、场景描述：单线程程序可以用try...catch捕获程序的异常，而在多线程程序的时候是无法使用try...catch捕获。

示例1：多线程发生异常，无法使用try...catch捕获问题

public class NoCaughtThread implements Runnable{

@Override

public void run() {

System.out.println(3 / 2);

System.out.println(3 / 0);

System.out.println(3 / 1);

}

public static void main(String[] args) {

try {

Thread thread = new Thread(new NoCaughtThread());

thread.start();

} catch (Exception e) {

System.out.println("==Exception: " + e.getMessage());

}

}

}

运行结果：

Exception in thread "Thread-0" java.lang.ArithmeticException: / by zero

at threadtest.NoCaughtThread.run(NoCaughtThread.java:7)

at java.lang.Thread.run(Thread.java:724)

显然这并非程序设定异常捕获，此时try...catch无法捕获线程的异常。此时，如果线程因为异常而终止执行将无法检测到异常。究其原因Thread类run()方法是不抛出任何检查型异常的，而自身却可能因为一个异常而被中止。

二、解决方式大致有两种：①在run()中设置对应的异常处理，主动方法来解决未检测异常；②Thread类API中提供Interface接口UncaughtExceptionHandler，该接口包含uncaughtException方法，它能检测出某个未捕获的异常而终结的情况；

示例2：主动的检测异常

import java.util.concurrent.ExecutorService;

import java.util.concurrent.Executors;

public class InitiativeCaught {

public static void main(String[] args) {

InitialtiveThread initialtiveThread = ew InitialtiveThread() ;

ExecutorService exec = Executors.newCachedThreadPool();

exec.execute(initialtiveThread);

exec.shutdown();

}

}

class InitialtiveThread implements Runnable {

@Override

public void run() {

Throwable thrown = null;

try {

System.out.println(3 / 2);

System.out.println(3 / 0);

System.out.println(3 / 1);

} catch (Throwable e) {

thrown = e;

} finally {

threadDeal(this, thrown);

}

}

public void threadDeal(Runnable r, Throwable t) {

System.out.println("==Exception: " + t.getMessage());

} 

}

运行结果：

1

==Exception: / by zero

此时是主动捕获异常并做处理，得到想要的结果。

示例3：Thread类API中提供UncaughtExceptionHandler接口捕获异常，要求检测线程异常，发生异常设置为重复调用三次之后结束线程。

import java.lang.Thread.UncaughtExceptionHandler;

import java.util.concurrent.ExecutorService;

import java.util.concurrent.Executors;

public class ThreadMonitor implements Runnable {

private int data;                   //可设置通过构造传参

private int control = 0;

private static final int MAX = 3;   //设置重试次数

public ThreadMonitor(int i) {

this.data = i;   

}

public ThreadMonitor() {

// TODO Auto-generated constructor stub

}

@Override

public void run() {

Thread.setDefaultUncaughtExceptionHandler(new UncaughtExceptionHandler() {

@Override

public void uncaughtException(Thread arg0, Throwable e) {

// TODO Auto-generated method stub

System.out.println("==Exception: " + e.getMessage());

String message = e.getMessage();

if( control==MAX ){

return ;

}else if( "ok".equals(message) ){

return ;

}else if ( "error".equals(message) ) {

new Thread() {

public void run() {

try {

System.out.println("开始睡眠。");

Thread.sleep(1 * 1000);

control++ ;

System.out.println("睡眠结束，control: "+ control);

myTask(data) ;

} catch (InterruptedException e) {

e.printStackTrace();

}

};

}.start();

}else{

return ;

}

}

}); 

myTask(data) ;

}

@SuppressWarnings("finally")

public void myTask(int data){

boolean flag = true ;

try {

System.out.println(4 / data);

} catch (Exception e) {

flag = false ;

} finally {

if( flag ){

throw new RuntimeException("ok");

}else{

throw new RuntimeException("error");

}

}

}

public static void main(String[] args) {

ExecutorService exec = Executors.newCachedThreadPool();

ThreadMonitor threadMonitor = new ThreadMonitor(0);

exec.execute(threadMonitor);

exec.shutdown();

}

}

运行结果：

==Exception: error

开始睡眠。

睡眠结束，control: 1

==Exception: error

开始睡眠。

睡眠结束，control: 2

==Exception: error

开始睡眠。

睡眠结束，control: 3

==Exception: error

此时，可以正常捕获线程因除数为零造成的中断。其中：

(1)在Thread类API中提供Interface接口UncaughtExceptionHandler，该接口包含一个uncaughtException方法，它能检测出某个由于未捕获的异常而终结的情况。定义如下：

UncaughtExceptionHandler接口: public static interface Thread.UncaughtExceptionHandler

uncaughtException方法：public void uncaughtException(Thread t, Throwable e)

(2) uncaughtException方法会捕获线程的异常，此时需要覆写该方法设定自定义的处理方式。

(3)设置UncaughtExceptionHandler异常处理：

方式一：通过Thread提供的静态static方法，设置默认异常处理：public static void setDefaultUncaughtExceptionHandler(Thread.UncaughtExceptionHandler ux)

方式二：通过方法：public void setUncaughtExceptionHandler(Thread.UncaughtExceptionHandler eh)

(4) UncaughtExceptionHandler异常处理需要设置在run()方法内，否则无法捕获到线程的异常。