[Android] FileInputStream跟踪

1. 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中分析的很好。

2. java.io.FileInputStream

import libcore.io.Libcore;
import libcore.io.Streams;
@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);
}

继续跟踪Streams.readSingleByte和IoBridge.read

3. 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);
    return (result != -1) ? buffer[0] & 0xff : -1;
}

这里用了InputStream.read(byte[], int, int)，实际上还是调用的FileInputStream.read(byte[], int, int)，最后还是调用IoBridge.read(byte[], int, int)

4. libcore.io.ioBridge

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

    /**
     * java.io thinks that a read at EOF is an error and should return -1, contrary to traditional
     * Unix practice where you'd read until you got 0 bytes (and any future read would return -1).
     */
    public static int read(FileDescriptor fd, byte[] bytes, int byteOffset, int byteCount) throws IOException {
        Arrays.checkOffsetAndCount(bytes.length, byteOffset, byteCount);
        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) {
                // We return 0 rather than throw if we try to read from an empty non-blocking pipe.
                return 0;
            }
            throw errnoException.rethrowAsIOException();
        }
    }

5. 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());
}

到这里有点断片了，BlockGuardOs里面还有一些调用，这里参考了http://blog.chinaunix.net/uid-26926660-id-3326678.html文章，read操作是由Posix实现的。

6. Libcore.io.Posix

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

public int read(FileDescriptor fd, ByteBuffer buffer) throws ErrnoException {
    if (buffer.isDirect()) {
        return readBytes(fd, buffer, buffer.position(), buffer.remaining());
    } else {
        return readBytes(fd, NioUtils.unsafeArray(buffer), NioUtils.unsafeArrayOffset(buffer) + buffer.position(), buffer.remaining());
    }
}

到这里就是native方法了。

7. 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);
    if (bytes.get() == NULL) {
        return -1;
    }
    int fd = jniGetFDFromFileDescriptor(env, javaFd);
    return throwIfMinusOne(env, "read", TEMP_FAILURE_RETRY(read(fd, bytes.get() + byteOffset, byteCount)));
}

第一：return throwIfMinusOne(env, "write", TEMP_FAILURE_RETRY(write(fd, bytes.get() + byteOffset, byteCount)));

throwIfMinusOne与TEMP_FAILURE_RETRY是宏定义。

write是<unistd.h>中的C语言库函数

第二：ScopedBytesRW

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

这个文件比较简单，仅仅include了JNIHelper.h，用来区分byte[]s与ByteBuffers的。

class ScopedBytesRW : public ScopedBytes<false> {
public:
    ScopedBytesRW(JNIEnv* env, jobject object) : ScopedBytes<false>(env, object) {}
    jbyte* get() {
        return mPtr;
    }
};

8. 本文链接

http://blog.csdn.net/xiaodongrush/article/details/10004997