1.前言
众所周知，Java是一门跨平台语言，针对不同的操作系统有不同的实现。本文从一个非常简单的api调用来看看Java具体是怎么做的.
2.源码分析
从FileInputStream.java中看到readBytes最后是native调用
/*** Reads a subarray as a sequence of bytes.* @param b the data to be written* @param off the start offset in the data* @param len the number of bytes that are written* @exception IOException If an I/O error has occurred.*/private native int readBytes(byte b[], int off, int len) throws IOException; // native调用/*** Reads up to <code>b.length</code> bytes of data from this input* stream into an array of bytes. This method blocks until some input* is available.** @param b the buffer into which the data is read.* @return the total number of bytes read into the buffer, or* <code>-1</code> if there is no more data because the end of* the file has been reached.* @exception IOException if an I/O error occurs.*/public int read(byte b[]) throws IOException {return readBytes(b, 0, b.length);}/** * Reads a subarray as a sequence of bytes. * @param b the data to be written * @param off the start offset in the data * @param len the number of bytes that are written * @exception IOException If an I/O error has occurred. */ private native int readBytes(byte b[], int off, int len) throws IOException; // native调用 /** * Reads up to <code>b.length</code> bytes of data from this input * stream into an array of bytes. This method blocks until some input * is available. * * @param b the buffer into which the data is read. * @return the total number of bytes read into the buffer, or * <code>-1</code> if there is no more data because the end of * the file has been reached. * @exception IOException if an I/O error occurs. */ public int read(byte b[]) throws IOException { return readBytes(b, 0, b.length); }/** * Reads a subarray as a sequence of bytes. * @param b the data to be written * @param off the start offset in the data * @param len the number of bytes that are written * @exception IOException If an I/O error has occurred. */ private native int readBytes(byte b[], int off, int len) throws IOException; // native调用 /** * Reads up to <code>b.length</code> bytes of data from this input * stream into an array of bytes. This method blocks until some input * is available. * * @param b the buffer into which the data is read. * @return the total number of bytes read into the buffer, or * <code>-1</code> if there is no more data because the end of * the file has been reached. * @exception IOException if an I/O error occurs. */ public int read(byte b[]) throws IOException { return readBytes(b, 0, b.length); }
从jdk源码中，我们找到FileInputStream.c(/jdk/src/share/native/java/io)，此文件定义了对应文件的native调用.
// FileInputStream.cJNIEXPORT jint JNICALLJava_java_io_FileInputStream_readBytes(JNIEnv *env, jobject this,jbyteArray bytes, jint off, jint len) {return readBytes(env, this, bytes, off, len, fis_fd);}// FileInputStream.c JNIEXPORT jint JNICALL Java_java_io_FileInputStream_readBytes(JNIEnv *env, jobject this, jbyteArray bytes, jint off, jint len) { return readBytes(env, this, bytes, off, len, fis_fd); }// FileInputStream.c JNIEXPORT jint JNICALL Java_java_io_FileInputStream_readBytes(JNIEnv *env, jobject this, jbyteArray bytes, jint off, jint len) { return readBytes(env, this, bytes, off, len, fis_fd); }
我们观察下当前的目录，可以看到java 对典型的四种unix like的系统（bsd, linux, macosx, solaris）， 以及windows 提供了特殊实现。share是公用部分。
在头部获取文件fd field （fd 是非负正整数，用来标识打开文件）
// FileInputStream.cJNIEXPORT void JNICALLJava_java_io_FileInputStream_initIDs(JNIEnv *env, jclass fdClass) {fis_fd = (*env)->GetFieldID(env, fdClass, "fd", "Ljava/io/FileDescriptor;"); /* fd field，后面用来获取 fd */}// FileInputStream.c JNIEXPORT void JNICALL Java_java_io_FileInputStream_initIDs(JNIEnv *env, jclass fdClass) { fis_fd = (*env)->GetFieldID(env, fdClass, "fd", "Ljava/io/FileDescriptor;"); /* fd field，后面用来获取 fd */ }// FileInputStream.c JNIEXPORT void JNICALL Java_java_io_FileInputStream_initIDs(JNIEnv *env, jclass fdClass) { fis_fd = (*env)->GetFieldID(env, fdClass, "fd", "Ljava/io/FileDescriptor;"); /* fd field，后面用来获取 fd */ }
继续调用readBytes
// ioutil.cjintreadBytes(JNIEnv *env, jobject this, jbyteArray bytes,jint off, jint len, jfieldID fid){jint nread;char stackBuf[BUF_SIZE];char *buf = NULL;FD fd;if (IS_NULL(bytes)) {JNU_ThrowNullPointerException(env, NULL);return -1;}if (outOfBounds(env, off, len, bytes)) { /* 越界判断 */JNU_ThrowByName(env, "java/lang/IndexOutOfBoundsException", NULL);return -1;}if (len == 0) {return 0;} else if (len > BUF_SIZE) {buf = malloc(len); /* 缓冲区不足，动态分配内存 */if (buf == NULL) {JNU_ThrowOutOfMemoryError(env, NULL);return 0;}} else {buf = stackBuf;}fd = GET_FD(this, fid); /* 获取fd */if (fd == -1) {JNU_ThrowIOException(env, "Stream Closed");nread = -1;} else {nread = IO_Read(fd, buf, len); /* 执行read，系统调用 */if (nread > 0) {(*env)->SetByteArrayRegion(env, bytes, off, nread, (jbyte *)buf);} else if (nread == -1) {JNU_ThrowIOExceptionWithLastError(env, "Read error");} else { /* EOF */nread = -1;}}if (buf != stackBuf) {free(buf); /* 失败释放内存 */}return nread;}// ioutil.c jint readBytes(JNIEnv *env, jobject this, jbyteArray bytes, jint off, jint len, jfieldID fid) { jint nread; char stackBuf[BUF_SIZE]; char *buf = NULL; FD fd; if (IS_NULL(bytes)) { JNU_ThrowNullPointerException(env, NULL); return -1; } if (outOfBounds(env, off, len, bytes)) { /* 越界判断 */ JNU_ThrowByName(env, "java/lang/IndexOutOfBoundsException", NULL); return -1; } if (len == 0) { return 0; } else if (len > BUF_SIZE) { buf = malloc(len); /* 缓冲区不足，动态分配内存 */ if (buf == NULL) { JNU_ThrowOutOfMemoryError(env, NULL); return 0; } } else { buf = stackBuf; } fd = GET_FD(this, fid); /* 获取fd */ if (fd == -1) { JNU_ThrowIOException(env, "Stream Closed"); nread = -1; } else { nread = IO_Read(fd, buf, len); /* 执行read，系统调用 */ if (nread > 0) { (*env)->SetByteArrayRegion(env, bytes, off, nread, (jbyte *)buf); } else if (nread == -1) { JNU_ThrowIOExceptionWithLastError(env, "Read error"); } else { /* EOF */ nread = -1; } } if (buf != stackBuf) { free(buf); /* 失败释放内存 */ } return nread; }// ioutil.c jint readBytes(JNIEnv *env, jobject this, jbyteArray bytes, jint off, jint len, jfieldID fid) { jint nread; char stackBuf[BUF_SIZE]; char *buf = NULL; FD fd; if (IS_NULL(bytes)) { JNU_ThrowNullPointerException(env, NULL); return -1; } if (outOfBounds(env, off, len, bytes)) { /* 越界判断 */ JNU_ThrowByName(env, "java/lang/IndexOutOfBoundsException", NULL); return -1; } if (len == 0) { return 0; } else if (len > BUF_SIZE) { buf = malloc(len); /* 缓冲区不足，动态分配内存 */ if (buf == NULL) { JNU_ThrowOutOfMemoryError(env, NULL); return 0; } } else { buf = stackBuf; } fd = GET_FD(this, fid); /* 获取fd */ if (fd == -1) { JNU_ThrowIOException(env, "Stream Closed"); nread = -1; } else { nread = IO_Read(fd, buf, len); /* 执行read，系统调用 */ if (nread > 0) { (*env)->SetByteArrayRegion(env, bytes, off, nread, (jbyte *)buf); } else if (nread == -1) { JNU_ThrowIOExceptionWithLastError(env, "Read error"); } else { /* EOF */ nread = -1; } } if (buf != stackBuf) { free(buf); /* 失败释放内存 */ } return nread; }
我们继续看看IO_Read的实现，是个宏定义
#define IO_Read handleReadhandleRead有两种实现
solaris实现：
// /jdk/src/solaris/native/java/io/io_util_md.cssize_thandleRead(FD fd, void *buf, jint len){ssize_t result;RESTARTABLE(read(fd, buf, len), result);return result;}/** Retry the operation if it is interrupted*/#define RESTARTABLE(_cmd, _result) do { \do { \_result = _cmd; \} while((_result == -1) && (errno == EINTR)); \ /* 如果是中断，则不断重试，避免进程调度等待*/} while(0)// /jdk/src/solaris/native/java/io/io_util_md.c ssize_t handleRead(FD fd, void *buf, jint len) { ssize_t result; RESTARTABLE(read(fd, buf, len), result); return result; } /* * Retry the operation if it is interrupted */ #define RESTARTABLE(_cmd, _result) do { \ do { \ _result = _cmd; \ } while((_result == -1) && (errno == EINTR)); \ /* 如果是中断，则不断重试，避免进程调度等待*/ } while(0)// /jdk/src/solaris/native/java/io/io_util_md.c ssize_t handleRead(FD fd, void *buf, jint len) { ssize_t result; RESTARTABLE(read(fd, buf, len), result); return result; } /* * Retry the operation if it is interrupted */ #define RESTARTABLE(_cmd, _result) do { \ do { \ _result = _cmd; \ } while((_result == -1) && (errno == EINTR)); \ /* 如果是中断，则不断重试，避免进程调度等待*/ } while(0)
read方法可以参考unix man page
windows实现：
// jdk/src/windows/native/java/io/io_util_md.cJNIEXPORTjinthandleRead(FD fd, void *buf, jint len){DWORD read = 0;BOOL result = 0;HANDLE h = (HANDLE)fd;if (h == INVALID_HANDLE_VALUE) {return -1;}result = ReadFile(h, /* File handle to read */buf, /* address to put data */len, /* number of bytes to read */&read, /* number of bytes read */NULL); /* no overlapped struct */if (result == 0) {int error = GetLastError();if (error == ERROR_BROKEN_PIPE) {return 0; /* EOF */}return -1;}return (jint)read;}// jdk/src/windows/native/java/io/io_util_md.c JNIEXPORT jint handleRead(FD fd, void *buf, jint len) { DWORD read = 0; BOOL result = 0; HANDLE h = (HANDLE)fd; if (h == INVALID_HANDLE_VALUE) { return -1; } result = ReadFile(h, /* File handle to read */ buf, /* address to put data */ len, /* number of bytes to read */ &read, /* number of bytes read */ NULL); /* no overlapped struct */ if (result == 0) { int error = GetLastError(); if (error == ERROR_BROKEN_PIPE) { return 0; /* EOF */ } return -1; } return (jint)read; }// jdk/src/windows/native/java/io/io_util_md.c JNIEXPORT jint handleRead(FD fd, void *buf, jint len) { DWORD read = 0; BOOL result = 0; HANDLE h = (HANDLE)fd; if (h == INVALID_HANDLE_VALUE) { return -1; } result = ReadFile(h, /* File handle to read */ buf, /* address to put data */ len, /* number of bytes to read */ &read, /* number of bytes read */ NULL); /* no overlapped struct */ if (result == 0) { int error = GetLastError(); if (error == ERROR_BROKEN_PIPE) { return 0; /* EOF */ } return -1; } return (jint)read; }
3.java异常初探
// jdk/src/share/native/common/jni_util.c/*** Throw a Java exception by name. Similar to SignalError.*/JNIEXPORT void JNICALLJNU_ThrowByName(JNIEnv *env, const char *name, const char *msg){jclass cls = (*env)->FindClass(env, name);if (cls != 0) /* Otherwise an exception has already been thrown */(*env)->ThrowNew(env, cls, msg); /* 调用JNI 接口*/}/* JNU_Throw common exceptions */JNIEXPORT void JNICALLJNU_ThrowNullPointerException(JNIEnv *env, const char *msg){JNU_ThrowByName(env, "java/lang/NullPointerException", msg);}// jdk/src/share/native/common/jni_util.c /** * Throw a Java exception by name. Similar to SignalError. */ JNIEXPORT void JNICALL JNU_ThrowByName(JNIEnv *env, const char *name, const char *msg) { jclass cls = (*env)->FindClass(env, name); if (cls != 0) /* Otherwise an exception has already been thrown */ (*env)->ThrowNew(env, cls, msg); /* 调用JNI 接口*/ } /* JNU_Throw common exceptions */ JNIEXPORT void JNICALL JNU_ThrowNullPointerException(JNIEnv *env, const char *msg) { JNU_ThrowByName(env, "java/lang/NullPointerException", msg); }// jdk/src/share/native/common/jni_util.c /** * Throw a Java exception by name. Similar to SignalError. */ JNIEXPORT void JNICALL JNU_ThrowByName(JNIEnv *env, const char *name, const char *msg) { jclass cls = (*env)->FindClass(env, name); if (cls != 0) /* Otherwise an exception has already been thrown */ (*env)->ThrowNew(env, cls, msg); /* 调用JNI 接口*/ } /* JNU_Throw common exceptions */ JNIEXPORT void JNICALL JNU_ThrowNullPointerException(JNIEnv *env, const char *msg) { JNU_ThrowByName(env, "java/lang/NullPointerException", msg); }
最后是调用JNI:
// hotspot/src/share/vm/prims/jni.hjint ThrowNew(jclass clazz, const char *msg) {return functions->ThrowNew(this, clazz, msg);}jint (JNICALL *ThrowNew)(JNIEnv *env, jclass clazz, const char *msg);// hotspot/src/share/vm/prims/jni.h jint ThrowNew(jclass clazz, const char *msg) { return functions->ThrowNew(this, clazz, msg); } jint (JNICALL *ThrowNew) (JNIEnv *env, jclass clazz, const char *msg);// hotspot/src/share/vm/prims/jni.h jint ThrowNew(jclass clazz, const char *msg) { return functions->ThrowNew(this, clazz, msg); } jint (JNICALL *ThrowNew) (JNIEnv *env, jclass clazz, const char *msg);
4.总结
很多高级语言，有着不同的编程范式，但是归根到底还是（c语言）系统调用，c语言能够在更低的层面做非常多的优化。如果我们了解了这些底层的系统调用，就能看到问题的本质。
本文没有对JNI 做深入分析，后续继续解析。
5.参考
https://man7.org/linux/man-pages/man2/read.2.html
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