[Android] FileInputStream跟踪

源起

    需要跟踪FileInputStream的Read的Nativie实现，开始走了弯路，Java工程下的FileInputStream实现与Android工程的实现不同。另外，http://blog.chinaunix.net/uid-26926660-id-3326678.html中分析的不错。

java.io.FileInputStream

   @Override public int read() throws IOException {

        return Streams.readSingleByte(this);

    }

    @Override public int read(byte[] buffer, int byteOffset, int byteCount) throws IOException {

        return IoBridge.read(fd, buffer, byteOffset, byteCount);

    }

---

libcore.io.Streams

http://grepcode.com/file/repo1.maven.org/maven2/com.google.okhttp/okhttp/20120626/libcore/io/Streams.java

public static int readSingleByte(InputStream in) throws IOException {

    byte[] buffer = new byte[1];

    int result = in.read(buffer, 0, 1); // in.read() -> FileInputStream.read -> IoBridge.read

    return (result != -1) ? buffer[0] & 0xff : -1;

}

最终还是调用IoBridge.read(FileDescriptor fd, byte[] bytes, int byteOffset, int byteCount) 

---

libcore.io.ioBridge

https://android.googlesource.com/platform/libcore/+/796f0d5a4e7b83c3efc5e587b6766977dc20b0c3/luni/src/main/java/libcore/io/IoBridge.java

    public static int read(FileDescriptor fd, byte[] bytes, int byteOffset, int byteCount) throws IOException {

        Arrays.checkOffsetAndCount(bytes.length, byteOffset, byteCount); 检查数组是否越界

        if (byteCount == 0) {

            return 0;

        }

        try {

            int readCount = Libcore.os.read(fd, bytes, byteOffset, byteCount);

            if (readCount == 0) {

                return -1;

            }

            return readCount;

        } catch (ErrnoException errnoException) {

            if (errnoException.errno == EAGAIN) { // Linux中非阻塞线程里面执行阻塞工作，会有这样的错误码，该错误码是native实现

                // We return 0 rather than throw if we try to read from an empty non-blocking pipe.

                return 0;

            }

            throw errnoException.rethrowAsIOException();

        }

    }

---

Java.util.Arrays

https://android.googlesource.com/platform/libcore/+/2bad9bff258de6275bd3847e5e9f3169b0a93c61/luni/src/main/java/java/util/Arrays.java

public static void checkOffsetAndCount(int arrayLength, int offset, int count) {

     if ((offset | count) < 0 || offset > arrayLength || arrayLength - offset < count) {

         throw new ArrayIndexOutOfBoundsException(arrayLength, offset, count);

    }

}

---

OsConstants.java

https://android.googlesource.com/platform/libcore/+/master/luni/src/main/java/libcore/io/OsConstants.java

public static final int EAGAIN = placeholder();

private static native void initConstants();

// A hack to avoid these constants being inlined by javac...

private static int placeholder() { return 0; }

// ...because we want to initialize them at runtime.

static {

   initConstants();

}

---

libcore_io_OsConstants.cpp

https://android.googlesource.com/platform/libcore/+/2bad9bff258de6275bd3847e5e9f3169b0a93c61/luni/src/main/native/libcore_io_OsConstants.cpp

#include <errno.h> http://baike.baidu.com/view/5806465.htm

#define EAGAIN 11 /* Try again */

---

libcore.io.Libcore

http://grepcode.com/file/repo1.maven.org/maven2/org.robovm/robovm-rt/0.0.2/libcore/io/Libcore.java#Libcore

package libcore.io;

public final class Libcore {

    private Libcore() { }

    public static Os os = new BlockGuardOs(new Posix());

}

---

BlockGuardOs

android / platform/libcore / 2bad9bff258de6275bd3847e5e9f3169b0a93c61 / . / luni / src / main / java / libcore / io / BlockGuardOs.java

@Override public int read(FileDescriptor fd, byte[] bytes, int byteOffset, int byteCount) throws ErrnoException {

    BlockGuard.getThreadPolicy().onReadFromDisk(); // BlockGuard. getThreadPolicy()是一个单例

    return os.read(fd, bytes, byteOffset, byteCount); // 使用的是Posix中的read

}

---

Libcore.io.Posix

https://android.googlesource.com/platform/libcore/+/6c9b5377550a9649ed1532d1fcdfeba116c74ead/luni/src/main/java/libcore/io/Posix.java

    

public int read(FileDescriptor fd, byte[] bytes, int byteOffset, int byteCount) throws ErrnoException {

        // This indirection isn't strictly necessary, but ensures that our public interface is type safe.

        return readBytes(fd, bytes, byteOffset, byteCount);

}

private native int readBytes(FileDescriptor fd, Object buffer, int offset, int byteCount) throws ErrnoException;

---

src/main/native/libcore_io_Posix.cpp

https://android.googlesource.com/platform/libcore/+/721ceca2a52a3c27aa751476c8562e1e68088e15/luni/src/main/native/libcore_io_Posix.cpp

#include <unistd.h>

#include "ScopeBytes.h"

static jint Posix_readBytes(JNIEnv* env, jobject, jobject javaFd, jobject javaBytes, jint byteOffset, jint byteCount) {

    ScopedBytesRW bytes(env, javaBytes); // ScopeBytes.h中有定义

    if (bytes.get() == NULL) {

        return -1;

    }

    int fd = jniGetFDFromFileDescriptor(env, javaFd);

    return throwIfMinusOne(env, "read", TEMP_FAILURE_RETRY(read(fd, bytes.get() + byteOffset, byteCount)));

}

1. ScopedBytesRW bytes(env, javaBytes); ScopeBytes.h中有定义

https://android.googlesource.com/platform/libnativehelper/+/3d9d2148155c2e0b3bf51cd548f58f93d1199a4e/include/nativehelper/ScopedBytes.h

2. throwIfMinusOne和TEMP_FAILURE_RETRY

3. read(fd, bytes.get() + byteOffset, byteCount)就是c的库函数

---

throwIfMinusOne

template <typename rc_t>

static rc_t throwIfMinusOne(JNIEnv* env, const char* name, rc_t rc) {

    if (rc == rc_t(-1)) {

        throwErrnoException(env, name);

    }

    return rc;

}

static void throwErrnoException(JNIEnv* env, const char* name) {

    int errnum = errno;

    jthrowable cause = NULL;

    if (env->ExceptionCheck()) {

        cause = env->ExceptionOccurred();

        env->ExceptionClear();

    }

    ScopedLocalRef<jstring> javaName(env, env->NewStringUTF(name));

    if (javaName.get() == NULL) {

        // Not really much we can do here. We're probably dead in the water,

        // but let's try to stumble on...

        env->ExceptionClear();

    }

    jobject exception;

    if (cause != NULL) {

        static jmethodID ctor = env->GetMethodID(JniConstants::errnoExceptionClass, "<init>",

                "(Ljava/lang/String;ILjava/lang/Throwable;)V");

        exception = env->NewObject(JniConstants::errnoExceptionClass, ctor,

                javaName.get(), errnum, cause);

    } else {

        static jmethodID ctor = env->GetMethodID(JniConstants::errnoExceptionClass, "<init>",

                "(Ljava/lang/String;I)V");

        exception = env->NewObject(JniConstants::errnoExceptionClass, ctor, javaName.get(), errnum);

    }

    env->Throw(reinterpret_cast<jthrowable>(exception));

}

---

TEMP_FAILURE_RETRY

unistd.h，http://sinf1252.info.ucl.ac.be/usr/include/unistd.h.html

# define TEMP_FAILURE_RETRY(expression) \

  (__extension__                                                              \

    ({ long int __result;                                                      \

       do __result = (long int) (expression);                                      \

       while (__result == -1L && errno == EINTR);                              \

       __result; }))

#endif

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本文链接

    http://blog.youkuaiyun.com/xiaodongrush/article/details/10004997