Binder 异常处理机制

Server 端的处理

如果Binder 程序示例之 Java 篇中的 sayHello 服务端方法在执行过程中抛出异常会怎么样？

    public void sayhello() throws android.os.RemoteException {
        cnt1++;
        Log.i(TAG, "sayhello : cnt = "+cnt1);
        throw new RuntimeException("testexception");
    }

为了搞清楚服务端的异常处理，我先回顾一下服务端收到远程调用时的调用栈：

如果 sayhello 函数内部抛出了异常，代码执行的流程如下：

//IHelloService 内部 stub 类成员方法
@Override public boolean onTransact(int code, android.os.Parcel data, android.os.Parcel reply, int flags) throws android.os.RemoteException
    {
      java.lang.String descriptor = DESCRIPTOR;
      switch (code)
      {
        //......
        case TRANSACTION_sayhello:
        {
          data.enforceInterface(descriptor);
          this.sayhello();
          reply.writeNoException();
          return true;
        }
        //省略其他无关 case
       //.....
      }
    }

在 IHelloService 内部 stub 类中的 onTransact 方法并没有捕获异常，我们抛出的异常会传递给上一层方法 execTransactInternal：

private boolean execTransactInternal(int code, long dataObj, long replyObj, int flags,
            int callingUid) {
        // Make sure the observer won't change while processing a transaction.
        final BinderInternal.Observer observer = sObserver;
        final CallSession callSession =
                observer != null ? observer.callStarted(this, code, UNSET_WORKSOURCE) : null;
        Parcel data = Parcel.obtain(dataObj);
        Parcel reply = Parcel.obtain(replyObj);
        // theoretically, we should call transact, which will call onTransact,
        // but all that does is rewind it, and we just got these from an IPC,
        // so we'll just call it directly.
        boolean res;
        // Log any exceptions as warnings, don't silently suppress them.
        // If the call was FLAG_ONEWAY then these exceptions disappear into the ether.
        final boolean tracingEnabled = Binder.isTracingEnabled();
        try {
            if (tracingEnabled) {
                final String transactionName = getTransactionName(code);
                Trace.traceBegin(Trace.TRACE_TAG_ALWAYS, getClass().getName() + ":"
                        + (transactionName != null ? transactionName : code));
            }
            //这里抛出异常
            res = onTransact(code, data, reply, flags);
            // 仅 Catch RemoteException RuntimeException 两个大类的异常
        } catch (RemoteException|RuntimeException e) {
            if (observer != null) {
                observer.callThrewException(callSession, e);
            }
            if (LOG_RUNTIME_EXCEPTION) {
                Log.w(TAG, "Caught a RuntimeException from the binder stub implementation.", e);
            }
            // oneway 直接打印错误信息
            if ((flags & FLAG_ONEWAY) != 0) {
                if (e instanceof RemoteException) {
                    Log.w(TAG, "Binder call failed.", e);
                } else {
                    Log.w(TAG, "Caught a RuntimeException from the binder stub implementation.", e);
                }
            } else {  // 非 oneway ，将异常写入 reply
                // Clear the parcel before writing the exception
                reply.setDataSize(0);
                reply.setDataPosition(0);
                // 将异常写入 reply
                reply.writeException(e);
            }
            res = true;
        } finally {
            if (tracingEnabled) {
                Trace.traceEnd(Trace.TRACE_TAG_ALWAYS);
            }
            if (observer != null) {
                // The parcel RPC headers have been called during onTransact so we can now access
                // the worksource uid from the parcel.
                final int workSourceUid = sWorkSourceProvider.resolveWorkSourceUid(
                        data.readCallingWorkSourceUid());
                observer.callEnded(callSession, data.dataSize(), reply.dataSize(), workSourceUid);
            }
        }
        checkParcel(this, code, reply, "Unreasonably large binder reply buffer");
        reply.recycle();
        data.recycle();

        // Just in case -- we are done with the IPC, so there should be no more strict
        // mode violations that have gathered for this thread.  Either they have been
        // parceled and are now in transport off to the caller, or we are returning back
        // to the main transaction loop to wait for another incoming transaction.  Either
        // way, strict mode begone!
        StrictMode.clearGatheredViolations();
        return res;
    }

接着我们来看 Exception 是怎么写入 Parcle：

    // frameworks/base/core/java/android/os/Parcel.java
    public final void writeException(@NonNull Exception e) {
        int code = 0;

        // Exception 转换成整型的 code
        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;
        }

        // 写入 Exception 对应的 code， 而不是 Exception 对象
        writeInt(code);
        StrictMode.clearGatheredViolations();
        if (code == 0) {
            if (e instanceof RuntimeException) {
                throw (RuntimeException) e;
            }
            throw new RuntimeException(e);
        }
        // 接着写入 Exception 的 msg 
        writeString(e.getMessage());
        final long timeNow = sParcelExceptionStackTrace ? SystemClock.elapsedRealtime() : 0;
        if (sParcelExceptionStackTrace && (timeNow - sLastWriteExceptionStackTrace
                > WRITE_EXCEPTION_STACK_TRACE_THRESHOLD_MS)) {
            sLastWriteExceptionStackTrace = timeNow;
            final int sizePosition = dataPosition();
            writeInt(0); // Header size will be filled in later
            StackTraceElement[] stackTrace = e.getStackTrace();
            final int truncatedSize = Math.min(stackTrace.length, 5);
            StringBuilder sb = new StringBuilder();
            for (int i = 0; i < truncatedSize; i++) {
                sb.append("\tat ").append(stackTrace[i]).append('\n');
            }
            writeString(sb.toString());
            final int payloadPosition = dataPosition();
            setDataPosition(sizePosition);
            // Write stack trace header size. Used in native side to skip the header
            writeInt(payloadPosition - sizePosition);
            setDataPosition(payloadPosition);
        } else {
            writeInt(0);
        }
        switch (code) {
            case EX_SERVICE_SPECIFIC:
                writeInt(((ServiceSpecificException) e).errorCode);
                break;
            case EX_PARCELABLE:
                // Write parceled exception prefixed by length
                final int sizePosition = dataPosition();
                writeInt(0);
                writeParcelable((Parcelable) e, Parcelable.PARCELABLE_WRITE_RETURN_VALUE);
                final int payloadPosition = dataPosition();
                setDataPosition(sizePosition);
                writeInt(payloadPosition - sizePosition);
                setDataPosition(payloadPosition);
                break;
        }
    }    

可以看出，这里是把 Exception 转换成 code 与 msg 写入 Parcel，而不是直接写 Exception 对象。

execTransactInternal 只处理了 RemoteException RuntimeException 两个大类的异常，其他未被 catch 的异常仍会被抛给上一级函数处理。

根据上文给出的服务端收到远程调用时的调用栈,异常会传递给 execTransact 函数，这个函数内部不会处理异常，接下来异常会进一步传递给 JNI层的 onTransact：

    // frameworks/base/core/jni/android_util_Binder.cpp
    status_t onTransact(
        uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags = 0) override
    {
        JNIEnv* env = javavm_to_jnienv(mVM);

        ALOGV("onTransact() on %p calling object %p in env %p vm %p\n", this, mObject, env, mVM);

        IPCThreadState* thread_state = IPCThreadState::self();
        const int32_t strict_policy_before = thread_state->getStrictModePolicy();

        //printf("Transact from %p to Java code sending: ", this);
        //data.print();
        //printf("\n");
        jboolean res = env->CallBooleanMethod(mObject, gBinderOffsets.mExecTransact,
            code, reinterpret_cast<jlong>(&data), reinterpret_cast<jlong>(reply), flags);

        //这里会检查剩余的异常
        if (env->ExceptionCheck()) {
            //直接打印异常信息
            ScopedLocalRef<jthrowable> excep(env, env->ExceptionOccurred());
            report_exception(env, excep.get(),
                "*** Uncaught remote exception!  "
                "(Exceptions are not yet supported across processes.)");
            res = JNI_FALSE;
        }

       //......
    }

剩余的异常都会在这里打印出来

Client 端的处理

接下来我们来看看 Client 端的处理：

      @Override public void sayhello() throws android.os.RemoteException
      {
        android.os.Parcel _data = android.os.Parcel.obtain();
        android.os.Parcel _reply = android.os.Parcel.obtain();
        try {
          _data.writeInterfaceToken(DESCRIPTOR);
          boolean _status = mRemote.transact(Stub.TRANSACTION_sayhello, _data, _reply, 0);
          if (!_status && getDefaultImpl() != null) {
            getDefaultImpl().sayhello();
            return;
          }
          _reply.readException();
        }
        finally {
          _reply.recycle();
          _data.recycle();
        }
      }

在客户端会从 Parcel reply 中读出异常信息：

    // frameworks/base/core/java/android/os/Parcel.java
    public final void readException() {
        // 读出 code
        int code = readExceptionCode();
        if (code != 0) {
            // 读出 msg
            String msg = readString();
            // 构建出 Exception 对象
            readException(code, msg);
        }
    }

    public final int readExceptionCode() {
        int code = readInt();
        if (code == EX_HAS_REPLY_HEADER) {
            int headerSize = readInt();
            if (headerSize == 0) {
                Log.e(TAG, "Unexpected zero-sized Parcel reply header.");
            } else {
                // Currently the only thing in the header is StrictMode stacks,
                // but discussions around event/RPC tracing suggest we might
                // put that here too.  If so, switch on sub-header tags here.
                // But for now, just parse out the StrictMode stuff.
                StrictMode.readAndHandleBinderCallViolations(this);
            }
            // And fat response headers are currently only used when
            // there are no exceptions, so return no error:
            return 0;
        }
        return code;
    }

    public final void readException(int code, String msg) {
        String remoteStackTrace = null;
        final int remoteStackPayloadSize = readInt();
        if (remoteStackPayloadSize > 0) {
            remoteStackTrace = readString();
        }

        Exception e = createException(code, msg);
        // Attach remote stack trace if availalble
        if (remoteStackTrace != null) {
            RemoteException cause = new RemoteException(
                    "Remote stack trace:\n" + remoteStackTrace, null, false, false);
            try {
                Throwable rootCause = ExceptionUtils.getRootCause(e);
                if (rootCause != null) {
                    rootCause.initCause(cause);
                }
            } catch (RuntimeException ex) {
                Log.e(TAG, "Cannot set cause " + cause + " for " + e, ex);
            }
        }
        //抛出异常
        SneakyThrow.sneakyThrow(e);
    }

        private Exception createException(int code, String msg) {
        switch (code) {
            case EX_PARCELABLE:
                if (readInt() > 0) {
                    return (Exception) readParcelable(Parcelable.class.getClassLoader());
                } else {
                    return new RuntimeException(msg + " [missing Parcelable]");
                }
            case EX_SECURITY:
                return new SecurityException(msg);
            case EX_BAD_PARCELABLE:
                return new BadParcelableException(msg);
            case EX_ILLEGAL_ARGUMENT:
                return new IllegalArgumentException(msg);
            case EX_NULL_POINTER:
                return new NullPointerException(msg);
            case EX_ILLEGAL_STATE:
                return new IllegalStateException(msg);
            case EX_NETWORK_MAIN_THREAD:
                return new NetworkOnMainThreadException();
            case EX_UNSUPPORTED_OPERATION:
                return new UnsupportedOperationException(msg);
            case EX_SERVICE_SPECIFIC:
                return new ServiceSpecificException(readInt(), msg);
        }
        return new RuntimeException("Unknown exception code: " + code
                + " msg " + msg);
    }

总的来说，就是读出 code 与 msg，然后构建新的 Exception 对象，最后抛出异常。