今天接到了个需求,需要用到跨进程抛异常。

怎样将异常从服务端抛到客户端

也就是说在Service端抛出的异常需要可以在Client端接收。印象中binder是可以传异常的,所以aidl直接走起:

// aidl文件interface ITestExceptionAidl { boolean testThrowException();}// service端实现public class AidlService extends Service { @Nullable @Override public IBinder onBind(Intent intent) { return new ITestExceptionAidl.Stub() { @Override public boolean testThrowException() throws RemoteException { if (true) { throw new RuntimeException("TestException"); } return true; } }; }}// client端实现bindService(intent, new ServiceConnection() { @Override public void onServiceConnected(ComponentName name, IBinder service) { ITestExceptionAidl aidl = ITestExceptionAidl.Stub.asInterface(service); try { aidl.testThrowException(); } catch (Exception e) { Log.e("testtest", "Exception", e); } } @Override public void onServiceDisconnected(ComponentName name) { }}, Context.BIND_AUTO_CREATE);

但是这个程序实际上运行起来是这样的:

01-01 05:31:55.475 4868 4880 E JavaBinder: *** Uncaught remote exception! (Exceptions are not yet supported across processes.)
01-01 05:31:55.475 4868 4880 E JavaBinder: java.lang.RuntimeException: TestException
01-01 05:31:55.475 4868 4880 E JavaBinder: at me.linjw.demo.ipcdemo.AidlService$1.testThrowException(AidlService.java:22)
01-01 05:31:55.475 4868 4880 E JavaBinder: at me.linjw.demo.ipcdemo.ITestExceptionAidl$Stub.onTransact(ITestExceptionAidl.java:48)
01-01 05:31:55.475 4868 4880 E JavaBinder: at android.os.Binder.execTransact(Binder.java:565)

看日志里面的ITestExceptionAidl$Stub.onTransact,也就是说在service端就已经被异常打断了,并没有传给client端,而且第一个大大的”Exceptions are not yet supported across processes.”是说异常不允许跨进程吗?但是我明明记得AIDL生成的代码里面就有向Parcel写入异常啊:

public boolean onTransact(int code, android.os.Parcel data, android.os.Parcel reply, int flags) throws android.os.RemoteException { switch (code) { case INTERFACE_TRANSACTION: { reply.writeString(DESCRIPTOR); return true; } case TRANSACTION_testThrowException: { data.enforceInterface(DESCRIPTOR); boolean _result = this.testThrowException(); reply.writeNoException(); // 这里写入的是没有抛出异常 reply.writeInt(((_result) ? (1) : (0))); return true; } } return super.onTransact(code, data, reply, flags);}

查找Parcel的源码,其实是有writeException方法的:

public final void writeException(Exception e) { int code = 0; if (e instanceof Parcelable && (e.getClass().getClassLoader() == Parcelable.class.getClassLoader())) { // We only send Parcelable exceptions that are in the // BootClassLoader to ensure that the receiver can unpack them code = EX_PARCELABLE; } else if (e instanceof SecurityException) { code = EX_SECURITY; } else if (e instanceof BadParcelableException) { code = EX_BAD_PARCELABLE; } else if (e instanceof IllegalArgumentException) { code = EX_ILLEGAL_ARGUMENT; } else if (e instanceof NullPointerException) { code = EX_NULL_POINTER; } else if (e instanceof IllegalStateException) { code = EX_ILLEGAL_STATE; } else if (e instanceof NetworkOnMainThreadException) { code = EX_NETWORK_MAIN_THREAD; } else if (e instanceof UnsupportedOperationException) { code = EX_UNSUPPORTED_OPERATION; } else if (e instanceof ServiceSpecificException) { code = EX_SERVICE_SPECIFIC; } writeInt(code); StrictMode.clearGatheredViolations(); if (code == 0) { if (e instanceof RuntimeException) { throw (RuntimeException) e; } throw new RuntimeException(e); } writeString(e.getMessage()); ...}

可以看到其实Parcel是支持写入异常的,但是只支持Parcelable的异常或者下面这几种异常:

• SecurityException
• BadParcelableException
• IllegalArgumentException
• NullPointerException
• IllegalStateException
• NetworkOnMainThreadException
• UnsupportedOperationException
• ServiceSpecificException
  

如果是普通的RuntimeException,这打断写入,继续抛出。

于是我们将RuntimeException改成它支持的UnsupportedOperationException试试:

// service端改成抛出UnsupportedOperationExceptionppublic class AidlService extends Service { @Nullable @Override public IBinder onBind(Intent intent) { return new ITestExceptionAidl.Stub() { @Override public boolean testThrowException() throws RemoteException { if (true) { throw new UnsupportedOperationException("TestException"); } return true; } }; }}// client端实现还是一样,不变bindService(intent, new ServiceConnection() { @Override public void onServiceConnected(ComponentName name, IBinder service) { ITestExceptionAidl aidl = ITestExceptionAidl.Stub.asInterface(service); try { aidl.testThrowException(); } catch (Exception e) { Log.e("testtest", "Exception", e); } } @Override public void onServiceDisconnected(ComponentName name) { }}, Context.BIND_AUTO_CREATE);

这样运行的话客户端就能捕获到异常:

01-01 05:49:46.770 19937 19937 E testtest: RemoteException
01-01 05:49:46.770 19937 19937 E testtest: java.lang.UnsupportedOperationException: TestException
01-01 05:49:46.770 19937 19937 E testtest: at android.os.Parcel.readException(Parcel.java:1728)
01-01 05:49:46.770 19937 19937 E testtest: at android.os.Parcel.readException(Parcel.java:1669)
01-01 05:49:46.770 19937 19937 E testtest: at me.linjw.demo.ipcdemo.ITestExceptionAidl$Stub$Proxy.testThrowException(ITestExceptionAidl.java:77)
01-01 05:49:46.770 19937 19937 E testtest: at me.linjw.demo.ipcdemo.MainActivity$3.onServiceConnected(MainActivity.java:132)
01-01 05:49:46.770 19937 19937 E testtest: at android.app.LoadedApk$ServiceDispatcher.doConnected(LoadedApk.java:1465)
01-01 05:49:46.770 19937 19937 E testtest: at android.app.LoadedApk$ServiceDispatcher$RunConnection.run(LoadedApk.java:1482)
01-01 05:49:46.770 19937 19937 E testtest: at android.os.Handler.handleCallback(Handler.java:751)
01-01 05:49:46.770 19937 19937 E testtest: at android.os.Handler.dispatchMessage(Handler.java:95)
01-01 05:49:46.770 19937 19937 E testtest: at android.os.Looper.loop(Looper.java:154)
01-01 05:49:46.770 19937 19937 E testtest: at android.app.ActivityThread.main(ActivityThread.java:6097)
01-01 05:49:46.770 19937 19937 E testtest: at java.lang.reflect.Method.invoke(Native Method)
01-01 05:49:46.770 19937 19937 E testtest: at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1052)
01-01 05:49:46.770 19937 19937 E testtest: at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:942)

跨进程传递异常的原理

好,知道了如何去跨进程传递异常之后,然后我们来看看异常到底是如何传递过去的。

让我们再来看看异常写入的代码:

// 有异常的情况public final void writeException(Exception e) { int code = 0; if (e instanceof Parcelable && (e.getClass().getClassLoader() == Parcelable.class.getClassLoader())) { // We only send Parcelable exceptions that are in the // BootClassLoader to ensure that the receiver can unpack them code = EX_PARCELABLE; } else if (e instanceof SecurityException) { code = EX_SECURITY; } else if (e instanceof BadParcelableException) { code = EX_BAD_PARCELABLE; } else if (e instanceof IllegalArgumentException) { code = EX_ILLEGAL_ARGUMENT; } else if (e instanceof NullPointerException) { code = EX_NULL_POINTER; } else if (e instanceof IllegalStateException) { code = EX_ILLEGAL_STATE; } else if (e instanceof NetworkOnMainThreadException) { code = EX_NETWORK_MAIN_THREAD; } else if (e instanceof UnsupportedOperationException) { code = EX_UNSUPPORTED_OPERATION; } else if (e instanceof ServiceSpecificException) { code = EX_SERVICE_SPECIFIC; } writeInt(code); StrictMode.clearGatheredViolations(); if (code == 0) { if (e instanceof RuntimeException) { throw (RuntimeException) e; } throw new RuntimeException(e); } writeString(e.getMessage()); // 之后还有一些写入堆栈的操作,比较多,这里可以不看}public final void writeNoException() { if (StrictMode.hasGatheredViolations()) { // 如果StrictMode收集到了写违规行为会走这里,我们可以不关注它 writeInt(EX_HAS_REPLY_HEADER); ... } else { // 一般情况下会走这里 writeInt(0); }}

这里给每种支持的异常都编了个号码,它会往Parcel写入。而0代表的是没有发生异常。然后再看看读取异常的代码:

public boolean testThrowException() throws android.os.RemoteException { android.os.Parcel _data = android.os.Parcel.obtain(); android.os.Parcel _reply = android.os.Parcel.obtain(); boolean _result; try { _data.writeInterfaceToken(DESCRIPTOR); mRemote.transact(Stub.TRANSACTION_testThrowException, _data, _reply, 0); _reply.readException(); _result = (0 != _reply.readInt()); } finally { _reply.recycle(); _data.recycle(); } return _result;}// android.os.Parcel.readExceptionpublic final void readException() { int code = readExceptionCode(); if (code != 0) { String msg = readString(); //在这个方法里面创建异常并且抛出 readException(code, msg); }}

然后这里有个需要注意的点就是异常必须是写在Parcel的头部的,也就是说如果没有异常,我们先要将0写到头部,然后再将返回值继续往后面写入。如果有异常,我们要先将异常编码写入头部,然后就不需要再写入返回值了。

这样,在客户端读取的时候读取的头部就能知道到底有没有异常,没有异常就继续读取返回值,有异常就将异常读取出来并且抛出。

// service端代码boolean _result = this.testThrowException();reply.writeNoException(); // 先写入异常reply.writeInt(((_result) ? (1) : (0))); // 再写入返回值// client端代码mRemote.transact(Stub.TRANSACTION_testThrowException, _data, _reply, 0);_reply.readException(); // 先读取异常,有异常的话readException方法里面会直接抛出_result = (0 != _reply.readInt()); // 再读取返回值

也就是Parcel的头部是一个标志位,标志了有异常或者无异常:

但是我们看到AIDL生成的代码都是写入的无异常,那我们抛出的异常是怎么传过去的呢?还记得这个打印吗?

01-01 05:31:55.475 4868 4880 E JavaBinder: *** Uncaught remote exception! (Exceptions are not yet supported across processes.)
01-01 05:31:55.475 4868 4880 E JavaBinder: java.lang.RuntimeException: TestException
01-01 05:31:55.475 4868 4880 E JavaBinder: at me.linjw.demo.ipcdemo.AidlService$1.testThrowException(AidlService.java:22)
01-01 05:31:55.475 4868 4880 E JavaBinder: at me.linjw.demo.ipcdemo.ITestExceptionAidl$Stub.onTransact(ITestExceptionAidl.java:48)
01-01 05:31:55.475 4868 4880 E JavaBinder: at android.os.Binder.execTransact(Binder.java:565)

我们去android.os.Binder.execTransact这里找找看, onTransact方法实际就是在这里被调用的

private boolean execTransact(int code, long dataObj, long replyObj, int flags) { Parcel data = Parcel.obtain(dataObj); Parcel reply = Parcel.obtain(replyObj); boolean res; try { res = onTransact(code, data, reply, flags); } catch (RemoteException|RuntimeException e) { ... reply.setDataPosition(0); reply.writeException(e); res = true; } catch (OutOfMemoryError e) { RuntimeException re = new RuntimeException("Out of memory", e); reply.setDataPosition(0); reply.writeException(re); res = true; } checkParcel(this, code, reply, "Unreasonably large binder reply buffer"); reply.recycle(); data.recycle(); return res;}

看,这里如果catch到了方法,也就是说我们服务端有抛出异常,就会在catch代码块里面先就Parcel的游标重置回0,然后往Parcel头部写入异常。

好,到了这里其实整个流程就差不多了,但是我发现我没有看到那个”Exceptions are not yet supported across processes.”字符串,这个不支持的提示又是哪里来的呢?

让我们再回忆下代码,在遇到不支持的异常类型的时候, writeException也会抛出异常:

public final void writeException(Exception e) { int code = 0; if (e instanceof Parcelable && (e.getClass().getClassLoader() == Parcelable.class.getClassLoader())) { // We only send Parcelable exceptions that are in the // BootClassLoader to ensure that the receiver can unpack them code = EX_PARCELABLE; } else if (e instanceof SecurityException) { code = EX_SECURITY; } else if (e instanceof BadParcelableException) { code = EX_BAD_PARCELABLE; } else if (e instanceof IllegalArgumentException) { code = EX_ILLEGAL_ARGUMENT; } else if (e instanceof NullPointerException) { code = EX_NULL_POINTER; } else if (e instanceof IllegalStateException) { code = EX_ILLEGAL_STATE; } else if (e instanceof NetworkOnMainThreadException) { code = EX_NETWORK_MAIN_THREAD; } else if (e instanceof UnsupportedOperationException) { code = EX_UNSUPPORTED_OPERATION; } else if (e instanceof ServiceSpecificException) { code = EX_SERVICE_SPECIFIC; } writeInt(code); StrictMode.clearGatheredViolations(); // code为0,代表不支持这种异常,继续把异常抛出或者创建RuntimeException抛出 if (code == 0) { if (e instanceof RuntimeException) { throw (RuntimeException) e; } throw new RuntimeException(e); } ...}

由于这个writeException,已经是在catch代码块里面运行的了,没有人再去catch它,于是就会打断这个流程,直接跳出。形成了一个Uncaught remote exception。

最后我们找到/frameworks/base/core/jni/android_util_Binder.cpp的onTransact方法,这里通过jni调到Java的execTransact方法,调用完之后进行ExceptionCheck,如果发现有异常的话就report_exception:

virtual status_t onTransact(uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags = 0) { JNIEnv* env = javavm_to_jnienv(mVM); IPCThreadState* thread_state = IPCThreadState::self(); const int32_t strict_policy_before = thread_state->getStrictModePolicy(); jboolean res = env->CallBooleanMethod(mObject, gBinderOffsets.mExecTransact, code, reinterpret_cast<jlong>(&data), reinterpret_cast<jlong>(reply), flags); if (env->ExceptionCheck()) { jthrowable excep = env->ExceptionOccurred(); // 就是这里啦 report_exception(env, excep, "*** Uncaught remote exception! " "(Exceptions are not yet supported across processes.)"); res = JNI_FALSE; env->DeleteLocalRef(excep); } ...}

以上就是本文的全部内容，希望对大家的学习有所帮助，也希望大家多多支持。