編輯:關於Android編程
1. 源起
需要跟蹤FileInputStream的Read的Nativie實現,開始走了彎路,Java工程下的FileInputStream實現與Android工程的實現不同。另外,http://blog.chinaunix.net/uid-26926660-id-3326678.html中分析的很好。
2. java.io.FileInputStream
[java]
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);
}
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);
}繼續跟蹤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
[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;
}
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;
}
這裡用了InputStream.read(byte[], int, int),實際上還是調用的FileInputStream.read(byte[], int, int),最後還是調用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]
/**
* 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();
}
}
/**
* 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
[java]
package libcore.io;
public final class Libcore {
private Libcore() { }
public static Os os = new BlockGuardOs(new Posix());
}
package libcore.io;
public final class Libcore {
private Libcore() { }
public static Os os = new BlockGuardOs(new Posix());
}
到這裡有點斷片了,BlockGuardOs裡面還有一些調用,
6. Libcore.io.Posix
https://android.googlesource.com/platform/libcore/+/6c9b5377550a9649ed1532d1fcdfeba116c74ead/luni/src/main/java/libcore/io/Posix.java
[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());
}
}
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());
}
}
到這裡就是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
[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)));
}
#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與TEMP_FAILURE_RETRY是宏定義。
write是<unistd.h>中的C語言庫函數
第二:ScopedBytesRW
https://android.googlesource.com/platform/libnativehelper/+/3d9d2148155c2e0b3bf51cd548f58f93d1199a4e/include/nativehelper/ScopedBytes.h
這個文件比較簡單,僅僅include了JNIHelper.h,用來區分byte[]s與ByteBuffers的。
[cpp]
class ScopedBytesRW : public ScopedBytes<false> {
public:
ScopedBytesRW(JNIEnv* env, jobject object) : ScopedBytes<false>(env, object) {}
jbyte* get() {
return mPtr;
}
};
class ScopedBytesRW : public ScopedBytes<false> {
public:
ScopedBytesRW(JNIEnv* env, jobject object) : ScopedBytes<false>(env, object) {}
jbyte* get() {
return mPtr;
}
};
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