[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

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