我们在开发的时候经常会遇到各种异常，当程序遇到异常，便会将异常信息抛到LogCat中，那这个过程是怎么实现的呢？
我们以一个例子开始：
import android.app.Activity;
import android.os.Bundle;public class MainActivity4 extends Activity {@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);throw new NullPointerException();}
}
这个程序一启动便会抛一个异常到Logcat中，就像这样：
10-10 16:44:16.200: W/dalvikvm(381): threadid=1: thread exiting with uncaught exception (group=0x41588d58)
10-10 16:44:16.200: W/System.err(381): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.sahadev.renren/com.sahadev.activitythemetest.MainActivity4}: java.lang.NullPointerException
10-10 16:44:16.200: W/System.err(381): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2263)
10-10 16:44:16.200: W/System.err(381): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2313)
10-10 16:44:16.200: W/System.err(381): at android.app.ActivityThread.access$800(ActivityThread.java:147)
10-10 16:44:16.200: W/System.err(381): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1226)
10-10 16:44:16.200: W/System.err(381): at android.os.Handler.dispatchMessage(Handler.java:102)
10-10 16:44:16.200: W/System.err(381): at android.os.Looper.loop(Looper.java:136)
10-10 16:44:16.200: W/System.err(381): at android.app.ActivityThread.main(ActivityThread.java:5137)
10-10 16:44:16.200: W/System.err(381): at java.lang.reflect.Method.invokeNative(Native Method)
10-10 16:44:16.200: W/System.err(381): at java.lang.reflect.Method.invoke(Method.java:515)
10-10 16:44:16.200: W/System.err(381): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:801)
10-10 16:44:16.200: W/System.err(381): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:617)
10-10 16:44:16.200: W/System.err(381): at dalvik.system.NativeStart.main(Native Method)
10-10 16:44:16.200: W/System.err(381): Caused by: java.lang.NullPointerException
10-10 16:44:16.200: W/System.err(381): at com.sahadev.activitythemetest.MainActivity4.onCreate(MainActivity4.java:12)
10-10 16:44:16.200: W/System.err(381): at android.app.Activity.performCreate(Activity.java:5231)
10-10 16:44:16.200: W/System.err(381): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
10-10 16:44:16.200: W/System.err(381): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2227)
10-10 16:44:16.200: W/System.err(381): ... 11 more
好，异常信息就会通过Logcat输出出来，接下来我们一起看一下它内部的工作原理:
首先：
我们知道通常我们在处理全局自定义异常的时候通常会这么写：
import java.lang.Thread.UncaughtExceptionHandler;public class YikaoGlobalCrashHandler implements UncaughtExceptionHandler {public YikaoGlobalCrashHandler() {super();Thread.setDefaultUncaughtExceptionHandler(this);}@Overridepublic void uncaughtException(Thread thread, Throwable ex) {}
}我们知道，如果我们不这么设定，系统是会自己处理异常的，那就一定有一个默认的异常处理对象，没错：
Thread.getDefaultUncaughtExceptionHandler();咱们从Java最基础层面看起，
我们的JAVA入口是：com.android.internal.os.RuntimeInit类的main方法，至于main方法在哪被调用，我们以后再讨论：
public static final void main(String[] argv) {if (argv.length == 2 && argv[1].equals("application")) {if (DEBUG) Slog.d(TAG, "RuntimeInit: Starting application");redirectLogStreams();} else {if (DEBUG) Slog.d(TAG, "RuntimeInit: Starting tool");}commonInit();/** Now that we're running in interpreted code, call back into native code* to run the system.*/nativeFinishInit();if (DEBUG) Slog.d(TAG, "Leaving RuntimeInit!");}我们关注的是commonInit方法：
private static final void commonInit() {if (DEBUG) Slog.d(TAG, "Entered RuntimeInit!");/* set default handler; this applies to all threads in the VM */Thread.setDefaultUncaughtExceptionHandler(new UncaughtHandler());/** Install a TimezoneGetter subclass for ZoneInfo.db*/TimezoneGetter.setInstance(new TimezoneGetter() {@Overridepublic String getId() {return SystemProperties.get("persist.sys.timezone");}});TimeZone.setDefault(null);/** Sets handler for java.util.logging to use Android log facilities.* The odd "new instance-and-then-throw-away" is a mirror of how* the "java.util.logging.config.class" system property works. We* can't use the system property here since the logger has almost* certainly already been initialized.*/LogManager.getLogManager().reset();new AndroidConfig();/** Sets the default HTTP User-Agent used by HttpURLConnection.*/String userAgent = getDefaultUserAgent();System.setProperty("http.agent", userAgent);/** Wire socket tagging to traffic stats.*/NetworkManagementSocketTagger.install();/** If we're running in an emulator launched with "-trace", put the* VM into emulator trace profiling mode so that the user can hit* F9/F10 at any time to capture traces. This has performance* consequences, so it's not something you want to do always.*/String trace = SystemProperties.get("ro.kernel.android.tracing");if (trace.equals("1")) {Slog.i(TAG, "NOTE: emulator trace profiling enabled");Debug.enableEmulatorTraceOutput();}initialized = true;}在我们代码的第二行看到：Thread.setDefaultUncaughtExceptionHandler(new UncaughtHandler());那这个UncaughtHandler类在哪被定义呢？我们还可以在RuntimeInit.java中找到答案：
/*** Use this to log a message when a thread exits due to an uncaught* exception. The framework catches these for the main threads, so* this should only matter for threads created by applications.*/private static class UncaughtHandler implements Thread.UncaughtExceptionHandler {public void uncaughtException(Thread t, Throwable e) {try {// Don't re-enter -- avoid infinite loops if crash-reporting crashes.if (mCrashing) return;mCrashing = true;if (mApplicationObject == null) {Clog_e(TAG, "*** FATAL EXCEPTION IN SYSTEM PROCESS: " + t.getName(), e);} else {StringBuilder message = new StringBuilder();message.append("FATAL EXCEPTION: ").append(t.getName()).append("\n");final String processName = ActivityThread.currentProcessName();if (processName != null) {message.append("Process: ").append(processName).append(", ");}message.append("PID: ").append(Process.myPid());Clog_e(TAG, message.toString(), e);}// Bring up crash dialog, wait for it to be dismissedActivityManagerNative.getDefault().handleApplicationCrash(mApplicationObject, new ApplicationErrorReport.CrashInfo(e));} catch (Throwable t2) {try {Clog_e(TAG, "Error reporting crash", t2);} catch (Throwable t3) {// Even Clog_e() fails! Oh well.}} finally {// Try everything to make sure this process goes away.Process.killProcess(Process.myPid());System.exit(10);}}}
Log.println_native(Log.LOG_ID_CRASH, Log.ERROR, tag,msg + '\n' + Log.getStackTraceString(tr));将Log日志输出到控制台。
接下来会调用
// Bring up crash dialog, wait for it to be dismissedActivityManagerNative.getDefault().handleApplicationCrash(mApplicationObject, new ApplicationErrorReport.CrashInfo(e));
最终它还会将我们的程序杀死退出：
// Try everything to make sure this process goes away.Process.killProcess(Process.myPid());System.exit(10);好这就是系统为我们提供的默认异常处理方法，接下来当然还有不少疑问：
1.RuntimeInit类的main方法是在哪被调用的。
2.throw new NullPointerException();这部分是怎么执行的。
3.Thread的defaultUncaughtHandler属性又是在哪被调用的。
4.等等
欢迎对这方面有兴趣的可以在评论区参与讨论，也有可能是我学的还太少。
