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