近做一個很簡單的函數，要兩個返回值，我打算有一個類封裝一下返回，後來領導要求使用StringBuilder返回，不用StringBuffer，因為局部變量StringBuffer最後還是要編譯為StringBuilder，因為StringBuilder沒有任何同步。
問了一下大家的看法，風南說InputStream 的int read(byte[] b) 從輸入流中讀取一定數量的字節並將其存儲在緩沖區數組 b 中，也是類似的，看來有必要研究一下IO了。
InputStream類中重要的函數，只有一個抽象的有用，
public abstract int read() throws IOException;
其他兩個read的意義不大。
public int read(byte b[], int off, int len) throws IOException 和
public int read(byte b[]) throws IOException {
return read(b, 0, b.length);
}
下面看它的子類FileInputStream的實現，
/* File Descriptor - handle to the open file */
private FileDescriptor fd;
這個很重要，這是文件句柄，C語言使用fopen函數的都知道。
/**
* Opens the specified file for reading.
* @param name the name of the file
*/
private native void open(String name) throws FileNotFoundException;
/**
* Reads a byte of data from this input stream. This method blocks
* if no input is yet available.
*
* @return the next byte of data, or <code>-1</code> if the end of the
* file is reached.
* @exception IOException if an I/O error occurs.
*/
public native int read() throws IOException;
/**
* 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;
子類修改了父的實現
/**
* 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 up to <code>len</code> bytes of data from this input stream
* into an array of bytes. If <code>len</code> is not zero, the method
* blocks until some input is available; otherwise, no
* bytes are read and <code>0</code> is returned.
*
* @param b the buffer into which the data is read.
* @param off the start offset in the destination array <code>b</code>
* @param len the maximum number of bytes 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 NullPointerException If <code>b</code> is <code>null</code>.
* @exception IndexOutOfBoundsException If <code>off</code> is negative,
* <code>len</code> is negative, or <code>len</code> is greater than
* <code>b.length - off</code>
* @exception IOException if an I/O error occurs.
*/
public int read(byte b[], int off, int len) throws IOException {
return readBytes(b, off, len);
}
看出readBytes是關鍵的所在，windows的native code中只有
JNIEXPORT void JNICALL
Java_java_io_FileInputStream_close0(JNIEnv *env, jobject this) {
handleClose(env, this, fis_fd);
}
是平台相關的。
其他的在share中
FileInputStream.c代碼
jfieldID fis_fd; /* id for jobject 'fd' in java.io.FileInputStream */
/**************************************************************
* static methods to store field ID's in initializers
*/
得到fd的值，這個很重要
JNIEXPORT void JNICALL
Java_java_io_FileInputStream_initIDs(JNIEnv *env, jclass fdClass) {
fis_fd = (*env)->GetFieldID(env, fdClass, "fd", "Ljava/io/FileDescriptor;");
}
/**************************************************************
* Input stream
*/
JNIEXPORT void JNICALL
Java_java_io_FileInputStream_open(JNIEnv *env, jobject this, jstring path) {
fileOpen(env, this, path, fis_fd, O_RDONLY);
}
JNIEXPORT jint JNICALL
Java_java_io_FileInputStream_read(JNIEnv *env, jobject this) {
return readSingle(env, this, fis_fd);
}
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);
}
JNIEXPORT jlong JNICALL
Java_java_io_FileInputStream_skip(JNIEnv *env, jobject this, jlong toSkip) {
jlong cur = jlong_zero;
jlong end = jlong_zero;
FD fd = GET_FD(this, fis_fd);
if ((cur = IO_Lseek(fd, (jlong)0, (jint)SEEK_CUR)) == -1) {
JNU_ThrowIOExceptionWithLastError(env, "Seek error");
} else if ((end = IO_Lseek(fd, toSkip, (jint)SEEK_CUR)) == -1) {
JNU_ThrowIOExceptionWithLastError(env, "Seek error");
}
return (end - cur);
}
JNIEXPORT jint JNICALL
Java_java_io_FileInputStream_available(JNIEnv *env, jobject this) {
jlong ret;
FD fd = GET_FD(this, fis_fd);
if (IO_Available(fd, &ret)) {
if (ret > INT_MAX) {
ret = (jlong) INT_MAX;
}
return jlong_to_jint(ret);
}
JNU_ThrowIOExceptionWithLastError(env, NULL);
return 0;
}
主要是readBytes的實現，打開io_util.c文件
/* IO helper functions */
int
readSingle(JNIEnv *env, jobject this, jfieldID fid) {
int nread;
char ret;
FD fd = GET_FD(this, fid);
nread = IO_Read(fd, &ret, 1);
if (nread == 0) { /* EOF */
return -1;
} else if (nread == JVM_IO_ERR) { /* error */
JNU_ThrowIOExceptionWithLastError(env, "Read error");
} else if (nread == JVM_IO_INTR) {
JNU_ThrowByName(env, "java/io/InterruptedIOException", 0);
}
return ret & 0xFF;
}
/* The maximum size of a stack-allocated buffer.
*/
#define BUF_SIZE 8192
int
readBytes(JNIEnv *env, jobject this, jbyteArray bytes,
jint off, jint len, jfieldID fid)
{
int nread, datalen;
char stackBuf[BUF_SIZE];
char *buf = 0;
FD fd;
if (IS_NULL(bytes)) {
JNU_ThrowNullPointerException(env, 0);
return -1;
}
datalen = (*env)->GetArrayLength(env, bytes);
if ((off < 0) || (off > datalen) ||
(len < 0) || ((off + len) > datalen) || ((off + len) < 0)) {
JNU_ThrowByName(env, "java/lang/IndexOutOfBoundsException", 0);
return -1;
}
if (len == 0) {
return 0;
} else if (len > BUF_SIZE) {
buf = malloc(len);
if (buf == 0) {
JNU_ThrowOutOfMemoryError(env, 0);
return 0;
}
} else {
buf = stackBuf;
}
fd = GET_FD(this, fid);
nread = IO_Read(fd, buf, len);
if (nread > 0) {
(*env)->SetByteArrayRegion(env, bytes, off, nread, (jbyte *)buf);
} else if (nread == JVM_IO_ERR) {
JNU_ThrowIOExceptionWithLastError(env, "Read error");
} else if (nread == JVM_IO_INTR) { /* EOF */
JNU_ThrowByName(env, "java/io/InterruptedIOException", 0);
} else { /* EOF */
nread = -1;
}
if (buf != stackBuf) {
free(buf);
}
return nread;
}
注意IO_Read在io_util_md.h做了手腳的，
#define IO_Read JVM_Read
其實就是JVM_Read，JVM_Read在hotspot源碼中定義的
//%note jvm_r6
return (jint)hpi::read(fd, buf, nbytes);
hpi定義了大量的函數
// HPI_FileInterface
static inline char* native_path(char *path);
static inline int file_type(const char *path);
static inline int open(const char *name, int mode, int perm);
static inline int close(int fd);
static inline jlong lseek(int fd, jlong off, int whence);
static inline int ftruncate(int fd, jlong length);
static inline int fsync(int fd);
static inline int available(int fd, jlong *bytes);
static inline size_t read(int fd, void *buf, unsigned int nBytes);
static inline size_t write(int fd, const void *buf, unsigned int nBytes);
static inline int fsize(int fd, jlong *size);
這就是和平台相關的代碼了。window下通過HPIDECL宏轉換的
