查看新闻/公告--[备忘]java调用socket的write时，发生了什么

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#include <windows.h>
#include <winsock2.h>
#include <ctype.h>
#include <stdio.h>
#include <stdlib.h>
#include <malloc.h>
#include <sys/types.h>

#include "java_net_SocketOutputStream.h"

#include "net_util.h"
#include "jni_util.h"

/************************************************************************
* SocketOutputStream
*/
static jfieldID IO_fd_fdID;

/*
* Class:     java_net_SocketOutputStream
* Method:    init
* Signature: ()V
*/
JNIEXPORT void JNICALL
Java_java_net_SocketOutputStream_init(JNIEnv *env, jclass cls) {
    IO_fd_fdID = NET_GetFileDescriptorID(env);
}

/*
* Class:     java_net_SocketOutputStream
* Method:    socketWrite
* Signature: (Ljava/io/FileDescriptor;[BII)V
*/
JNIEXPORT void JNICALL
Java_java_net_SocketOutputStream_socketWrite0(JNIEnv *env, jobject this,
                                              jobject fdObj, jbyteArray data,
                                              jint off, jint len) {
    char *bufP;
    char BUF[MAX_BUFFER_LEN];
    int buflen;
    int fd;

    if (IS_NULL(fdObj)) {
        JNU_ThrowByName(env, JNU_JAVANETPKG "SocketException", "Socket closed");
        return;
    } else {
        fd = (*env)->GetIntField(env, fdObj, IO_fd_fdID);
    }
    if (IS_NULL(data)) {
        JNU_ThrowNullPointerException(env, "data argument");
        return;
    }

    /*
     * Use stack allocate buffer if possible. For large sizes we allocate
     * an intermediate buffer from the heap (up to a maximum). If heap is
     * unavailable just use our stack buffer.
     */
    if (len <= MAX_BUFFER_LEN) {
        bufP = BUF;
        buflen = MAX_BUFFER_LEN;
    } else {
        buflen = min(MAX_HEAP_BUFFER_LEN, len);
        bufP = (char *)malloc((size_t)buflen);
        if (bufP == NULL) {
            bufP = BUF;
            buflen = MAX_BUFFER_LEN;
        }
    }

    while(len > 0) {
        int loff = 0;
        int chunkLen = min(buflen, len);
        int llen = chunkLen;
        int retry = 0;

        (*env)->GetByteArrayRegion(env, data, off, chunkLen, (jbyte *)bufP);
        if ((*env)->ExceptionCheck(env)) {
            break;
        } else {
            while(llen > 0) {
                int n = send(fd, bufP + loff, llen, 0);
                if (n > 0) {
                    llen -= n;
                    loff += n;
                    continue;
                }

                /*
                 * Due to a bug in Windows Sockets (observed on NT and Windows
                 * 2000) it may be necessary to retry the send. The issue is that
                 * on blocking sockets send/WSASend is supposed to block if there
                 * is insufficient buffer space available. If there are a large
                 * number of threads blocked on write due to congestion then it's
                 * possile to hit the NT/2000 bug whereby send returns WSAENOBUFS.
                 * The workaround we use is to retry the send. If we have a
                 * large buffer to send (>2k) then we retry with a maximum of
                 * 2k buffer. If we hit the issue with <=2k buffer then we backoff
                 * for 1 second and retry again. We repeat this up to a reasonable
                 * limit before bailing out and throwing an exception. In load
                 * conditions we've observed that the send will succeed after 2-3
                 * attempts but this depends on network buffers associated with
                 * other sockets draining.
                 */
                if (WSAGetLastError() == WSAENOBUFS) {
                    if (llen > MAX_BUFFER_LEN) {
                        buflen = MAX_BUFFER_LEN;
                        chunkLen = MAX_BUFFER_LEN;
                        llen = MAX_BUFFER_LEN;
                        continue;
                    }
                    if (retry >= 30) {
                        JNU_ThrowByName(env, JNU_JAVANETPKG "SocketException",
                            "No buffer space available - exhausted attempts to queue buffer");
                        if (bufP != BUF) {
                            free(bufP);
                        }
                        return;
                    }
                    Sleep(1000);
                    retry++;
                    continue;
                }

                /*
                 * Send failed - can be caused by close or write error.
                 */
                if (WSAGetLastError() == WSAENOTSOCK) {
                    JNU_ThrowByName(env, JNU_JAVANETPKG "SocketException", "Socket closed");
                } else {
                    NET_ThrowCurrent(env, "socket write error");
                }
                if (bufP != BUF) {
                    free(bufP);
                }
                return;
            }
            len -= chunkLen;
            off += chunkLen;
        }
    }

    if (bufP != BUF) {
        free(bufP);
    }
}