OutPutStream的write方法的返回值为Void，但是操作系统的write接口返回值为int，下面以FileOutputStream为例看下是怎么封装的。
1.FileOutputStream.write

public void write(byte b[]) throws IOException {
        writeBytes(b, 0, b.length, append);
    }

2. FileOutputStream.writeBytes

 /**
     * Writes a sub array 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
     * @param append {@code true} to first advance the position to the
     *     end of file
     * @exception IOException If an I/O error has occurred.
     */
    private native void writeBytes(byte b[], int off, int len, boolean append)
        throws IOException;

3. 最后调用的native方法
   文件名：/jdk/src/share/native/java/io/io_util.c

void
writeBytes(JNIEnv *env, jobject this, jbyteArray bytes,
           jint off, jint len, jboolean append, jfieldID fid)
{
    jint n;
    char stackBuf[BUF_SIZE];
    char *buf = NULL;
    FD fd;

    if (IS_NULL(bytes)) {
        JNU_ThrowNullPointerException(env, NULL);
        return;
    }

    if (outOfBounds(env, off, len, bytes)) {
        JNU_ThrowByName(env, "java/lang/IndexOutOfBoundsException", NULL);
        return;
    }

    if (len == 0) {
        return;
    } else if (len > BUF_SIZE) {
        buf = malloc(len);
        if (buf == NULL) {
            JNU_ThrowOutOfMemoryError(env, NULL);
            return;
        }
    } else {
        buf = stackBuf;
    }

    (*env)->GetByteArrayRegion(env, bytes, off, len, (jbyte *)buf);

    if (!(*env)->ExceptionOccurred(env)) {
        off = 0;
        while (len > 0) {
            fd = GET_FD(this, fid);
            if (fd == -1) {
                JNU_ThrowIOException(env, "Stream Closed");
                break;
            }
            if (append == JNI_TRUE) {
                n = IO_Append(fd, buf+off, len);
            } else {
                n = IO_Write(fd, buf+off, len);
            }
            if (n == -1) {
                JNU_ThrowIOExceptionWithLastError(env, "Write error");
                break;
            }
            off += n;
            len -= n;
        }
    }
    if (buf != stackBuf) {
        free(buf);
    }
}