Java的readBytes是怎么实现的？

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);
    }

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.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.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.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 handleRead

handleRead有两种实现

solaris实现：

// /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.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 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.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