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中分析的不錯。
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.Streamshttp://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;}最終還是調用IoBridge.read(FileDescriptor fd, byte[] bytes, int byteOffset, int byteCount)
libcore.io.ioBridgehttps://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); 檢查數組是否越界 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中非阻塞線程裡面執行阻塞工作，會有這樣的錯誤碼，該錯誤碼是native實現 // We return 0 rather than throw if we try to read from an empty non-blocking pipe. return 0; } throw errnoException.rethrowAsIOException(); } }
Java.util.Arrayshttps://android.googlesource.com/platform/libcore/+/2bad9bff258de6275bd3847e5e9f3169b0a93c61/luni/src/main/java/java/util/Arrays.javapublic 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.javahttps://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.cpphttps://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.Libcorehttp://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());}
BlockGuardOsandroid / 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()是一個單例 return os.read(fd, bytes, byteOffset, byteCount); // 使用的是Posix中的read}
Libcore.io.Posixhttps://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中有定義 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中有定義https://android.googlesource.com/platform/libnativehelper/+/3d9d2148155c2e0b3bf51cd548f58f93d1199a4e/include/nativehelper/ScopedBytes.h2. throwIfMinusOne和TEMP_FAILURE_RETRY3. read(fd, bytes.get() + byteOffset, byteCount)就是c的庫函數
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
本文連結
http://blog.csdn.net/xiaodongrush/article/details/10004997
