从源码角度分析Android系统的异常捕获机制是如何运行的-阿里云开发者社区

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从源码角度分析Android系统的异常捕获机制是如何运行的

简介: 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.

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 {

	@Override
	protected 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);

	}

	@Override
	public void uncaughtException(Thread thread, Throwable ex) {

	}
}
通过这样的方式,我们便可以使程序在遇到异常的时候回调我们的对象实例,然后调用我们的uncaughtException方法。

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();
通过这个方法会返回一个系统默认的UncaughtExceptionHandler对象,那么这个对象是在哪被设置进去的呢?我们从源代码里面找答案:

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() {
            @Override
            public 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 dismissed
                ActivityManagerNative.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);
            }
        }
    }


我们看到代码中使用StringBuilder的message对象对基本信息进行了组合,然后调用Clog_e方法,Clog_e方法通过

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 dismissed
 ActivityManagerNative.getDefault().handleApplicationCrash(mApplicationObject, new ApplicationErrorReport.CrashInfo(e));
方法将我们的崩溃的Dialog显示出来,就像这样:

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。

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