1.需要UncaughtExceptionHandler的理由
- 主线程中可以很方便地发现异常，如打印堆栈信息，而子线程不能，代码示例如ExceptionInChildThread类所示。
- 子线程无法用传统的try catch方式捕获异常，代码示例如CannotCatchDirectly类所示。
- 不捕获异常的话，子线程抛出异常停止运行，破坏系统的健壮性。
/**
* 描述:研究单线程抛出时的处理情况
*/
public class ExceptionInChildThread implements Runnable {
@Override
public void run() {
throw new RuntimeException();
}
public static void main(String[] args) {
ExceptionInChildThread exceptionInChildThread = new ExceptionInChildThread();
Thread t = new Thread(exceptionInChildThread);
t.start();
for (int i = 1; i < 10; i++) {
System.out.println("正在打印" + i);
}
}
}
程序执行结果如下所示，可以看到堆栈信息被散乱输出到控制台，不是很便利。
Exception in thread "Thread-0" java.lang.RuntimeException
正在打印1
正在打印2
at com.leichuangkj.threads.ExceptionInChildThread.run(ExceptionInChildThread.java:6)
正在打印3
at java.lang.Thread.run(Thread.java:748)
正在打印4
正在打印5
正在打印6
正在打印7
正在打印8
正在打印9
/**
* 描述:证明线程的异常不能用传统方法捕获
* 1.不加try catch抛出4个异常，都带线程名字
* 2.加了try catch，期望捕获到第一个线程的异常，线程234不应该被捕获，期待打印出Caught Exception
* 3.执行时发现，根本没有Caught Exception，线程234依然运行并且抛出异常
*/
public class CannotCatchDirectly implements Runnable {
@Override
public void run() {
throw new RuntimeException();
}
public static void main(String[] args) throws InterruptedException {
CannotCatchDirectly cannotCatchDirectly = new CannotCatchDirectly();
try {
Thread t1 = new Thread(cannotCatchDirectly, "MyThread1");
t1.start();
Thread.sleep(1000);
Thread t2 = new Thread(cannotCatchDirectly, "MyThread2");
t2.start();
Thread.sleep(1000);
Thread t3 = new Thread(cannotCatchDirectly, "MyThread3");
t3.start();
Thread.sleep(1000);
Thread t4 = new Thread(cannotCatchDirectly, "MyThread4");
t4.start();
Thread.sleep(1000);
} catch (RuntimeException exception) {
System.out.println(Thread.currentThread().getName() + "抛出异常");
}
}
}
程序执行结果如下所示，当MyThread1抛出异常时，应当进入catch分支，并终止程序运行。但实际上没有，程序继续运行，4个子线程分别抛出异常，这说明使用try catch没有捕获到子线程异常。
Exception in thread "MyThread1" java.lang.RuntimeException
at com.leichuangkj.threads.CannotCatchDirectly.run(CannotCatchDirectly.java:6)
at java.lang.Thread.run(Thread.java:748)
Exception in thread "MyThread2" java.lang.RuntimeException
at com.leichuangkj.threads.CannotCatchDirectly.run(CannotCatchDirectly.java:6)
at java.lang.Thread.run(Thread.java:748)
Exception in thread "MyThread3" java.lang.RuntimeException
at com.leichuangkj.threads.CannotCatchDirectly.run(CannotCatchDirectly.java:6)
at java.lang.Thread.run(Thread.java:748)
Exception in thread "MyThread4" java.lang.RuntimeException
at com.leichuangkj.threads.CannotCatchDirectly.run(CannotCatchDirectly.java:6)
at java.lang.Thread.run(Thread.java:748)
2.UncaughtExceptionHandler源码分析
public class ThreadGroup implements Thread.UncaughtExceptionHandler {
public void uncaughtException(Thread t, Throwable e) {
//默认情况下parent是null
if (parent != null) {
parent.uncaughtException(t, e);
} else {
//设置全局handler进行处理
Thread.UncaughtExceptionHandler ueh = Thread.getDefaultUncaughtExceptionHandler();
if (ueh != null) {
ueh.uncaughtException(t, e);
} else if (!(e instanceof ThreadDeath)) {
//全局handler不存在则输出异常堆栈
System.err.print("Exception in thread \"" + t.getName() + "\" ");
e.printStackTrace(System.err);
}
}
}
}
3.全局异常处理器
/**
* 描述:自定义UncaughtExceptionHandler
*/
public class MyUncaughtExceptionHandler implements Thread.UncaughtExceptionHandler {
private String name;
MyUncaughtExceptionHandler(String name){
this.name = name;
}
@Override
public void uncaughtException(Thread t, Throwable e) {
System.out.println(t.getName() + "捕获异常,异常信息" + e.toString());
}
}
/**
* 描述:使用自定义的UncaughtExceptionHandler
*/
class UseOwnUncaughtExceptionHandler implements Runnable {
@Override
public void run() {
throw new RuntimeException();
}
public static void main(String[] args) throws InterruptedException {
Thread.setDefaultUncaughtExceptionHandler(new MyUncaughtExceptionHandler("捕获器"));
UseOwnUncaughtExceptionHandler useOwnUncaughtExceptionHandler = new UseOwnUncaughtExceptionHandler();
Thread t1 = new Thread(useOwnUncaughtExceptionHandler, "MyThread1");
t1.start();
Thread.sleep(1000);
Thread t2 = new Thread(useOwnUncaughtExceptionHandler, "MyThread2");
t2.start();
Thread.sleep(1000);
Thread t3 = new Thread(useOwnUncaughtExceptionHandler, "MyThread3");
t3.start();
Thread.sleep(1000);
Thread t4 = new Thread(useOwnUncaughtExceptionHandler, "MyThread4");
t4.start();
Thread.sleep(1000);
}
}
程序执行结果如下所示,4个子线程的抛出的异常都被捕获。
MyThread1捕获异常,异常信息java.lang.RuntimeException
MyThread2捕获异常,异常信息java.lang.RuntimeException
MyThread3捕获异常,异常信息java.lang.RuntimeException
MyThread4捕获异常,异常信息java.lang.RuntimeException