根據ｊａｖａ執行緒的本質，當一個執行緒丟擲異常時，在主執行緒中加try catch 是無法捕獲到其丟擲的異常的，如下面程式碼所示：

   private static final String TAG = "MainActivity";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }

    public
 
 void onClickBtn(View view) {
        try {
            Thread thread = new Thread(new ExceptionThread());
            ExecutorService service = Executors.newCachedThreadPool();
            service.execute(thread);
        } catch (Exception e) {
            Log.d(TAG, "onClickBtn: catch the excepiton"
 
);
            e.printStackTrace();
        }
    }

    class ExceptionThread implements Runnable {

        @Override
        public void run() {
            Log.d(TAG, "run: start to throw exception");
            throw new RuntimeException();
        }
    }

點選按鈕應用就會crash，異常log如下：

2018-11-12 16:32:08.106 32514-32545/com.air.testexceptioncatchinthead D/MainActivity: run: start to throw exception
    
    --------- beginning of crash
2018-11-12 16:32:08.113 32514-32545/com.air.testexceptioncatchinthead E/AndroidRuntime: FATAL EXCEPTION: pool-1-thread-1
    Process: com.air.testexceptioncatchinthead, PID: 32514
    java.lang.RuntimeException
        at com.air.testexceptioncatchinthead.MainActivity$ExceptionThread.run(MainActivity.java:37)
        at java.lang.Thread.run(Thread.java:764)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1167)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:641)
        at java.lang.Thread.run(Thread.java:764)
 

從上面實驗中可以看到，在有多個執行緒時，另外一個執行緒是無法捕獲到另外一個執行緒丟擲的異常的。

那麼這種異常如何捕獲呢，java 在JDK5中引入了UncaughtExceptionHandler介面，實現如下：

class MyUncaughtExceptionHandler implements Thread.UncaughtExceptionHandler {
        @Override
        public void uncaughtException(Thread t, Throwable e) {
            Log.d(TAG, "uncaughtException: caught " + t + e.toString());
            e.printStackTrace();
        }
    }
public void onClickBtn(View view) {
        Thread.setDefaultUncaughtExceptionHandler(new MyUncaughtExceptionHandler());
        new Thread(new ExceptionThread()).start();
    }

此時點選，就會發現app已經不會crash了，完整程式碼如下：

public class MainActivity extends AppCompatActivity {
    private static final String TAG = "MainActivity";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }

    public void onClickBtn(View view) {
        Thread.setDefaultUncaughtExceptionHandler(new MyUncaughtExceptionHandler());
        new Thread(new ExceptionThread()).start();
    }

    class ExceptionThread implements Runnable {

        @Override
        public void run() {
            Log.d(TAG, "run: start to throw exception");
            throw new NullPointerException();
        }
    }

    class MyUncaughtExceptionHandler implements Thread.UncaughtExceptionHandler {
        @Override
        public void uncaughtException(Thread t, Throwable e) {
            Log.d(TAG, "uncaughtException: caught " + t + e.toString());
            e.printStackTrace();
        }
    }
} 

注意：JVM設計源於這樣一種理念，執行緒是獨立執行的程式碼片斷，執行緒的問題應該由執行緒自己來解決，而不要委託到外部。基於這樣的理念，在java中執行緒方法的異常都應該線上程內部處理掉，這種外部捕獲異常的方案通常情況下不應該使用