关于java io 的 write 与 操作系统

新文章连接

熟悉linux 的同学 对 sync、fsync与fdatasync 应该不会陌生
如果不熟悉可以看下这篇总结不错的文章http://www.cnblogs.com/bhlsheji/p/5222271.html

但是java和其关系如何呢？
先上个栗子

@Test    public void file(){        try (FileOutputStream fo = new FileOutputStream("D:\\temp\\file",true)) {            for(int i=0;i<17;i++){                fo.write(String.format("Line : %s \n",i).getBytes());                fo.flush();                if(i%3==0){                    fo.getFD().sync();                }                TimeUnit.MILLISECONDS.sleep(1000);                System.out.println(i);            }        } catch (IOException e) {            e.printStackTrace();        } catch (InterruptedException e) {            e.printStackTrace();        }    }

一般来说我们会用flush，但是在需要确保数据落盘的场景，我们如何保证呢？java的实现是哪个级别的sync?

FileDescriptor 内的sync声明如下

public native void sync() throws SyncFailedException;

然后参阅openjdk的实现
openjdk/jdk/src/java.base/unix/native/libjava/FileDescriptor_md.c

JNIEXPORT void JNICALLJava_java_io_FileDescriptor_sync(JNIEnv *env, jobject this) {    FD fd = THIS_FD(this);    if (IO_Sync(fd) == -1) {        JNU_ThrowByName(env, "java/io/SyncFailedException", "sync failed");    }}

可以看到jvm直接调用了IO_Sync方法，而该方法的定义位于
openjdk/jdk/src/java.base/unix/native/libjava/io_util_md.h

/* * Copyright (c) 2003, 2015, Oracle and/or its affiliates. All rights reserved. * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. * * This code is free software; you can redistribute it and/or modify it * under the terms of the GNU General Public License version 2 only, as * published by the Free Software Foundation.  Oracle designates this * particular file as subject to the "Classpath" exception as provided * by Oracle in the LICENSE file that accompanied this code. * * This code is distributed in the hope that it will be useful, but WITHOUT * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License * version 2 for more details (a copy is included in the LICENSE file that * accompanied this code). * * You should have received a copy of the GNU General Public License version * 2 along with this work; if not, write to the Free Software Foundation, * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. * * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA * or visit www.oracle.com if you need additional information or have any * questions. */#include "jni_util.h"/* * Macros to use the right data type for file descriptors */#define FD jint/* * Prototypes for functions in io_util_md.c called from io_util.c, * FileDescriptor.c, FileInputStream.c, FileOutputStream.c, * UnixFileSystem_md.c */ssize_t handleWrite(FD fd, const void *buf, jint len);ssize_t handleRead(FD fd, void *buf, jint len);jint handleAvailable(FD fd, jlong *pbytes);jint handleSetLength(FD fd, jlong length);jlong handleGetLength(FD fd);FD handleOpen(const char *path, int oflag, int mode);/* * Macros to set/get fd from the java.io.FileDescriptor.  These * macros rely on having an appropriately defined 'this' object * within the scope in which they're used. * If GetObjectField returns null, SET_FD will stop and GET_FD * will simply return -1 to avoid crashing VM. */#define SET_FD(this, fd, fid) \    if ((*env)->GetObjectField(env, (this), (fid)) != NULL) \        (*env)->SetIntField(env, (*env)->GetObjectField(env, (this), (fid)),IO_fd_fdID, (fd))#define GET_FD(this, fid) \    (*env)->GetObjectField(env, (this), (fid)) == NULL ? \        -1 : (*env)->GetIntField(env, (*env)->GetObjectField(env, (this), (fid)), IO_fd_fdID)/* * Macros to set/get fd when inside java.io.FileDescriptor */#define THIS_FD(obj) (*env)->GetIntField(env, obj, IO_fd_fdID)/* * Route the routines */#define IO_Sync fsync#define IO_Read handleRead#define IO_Write handleWrite#define IO_Append handleWrite#define IO_Available handleAvailable#define IO_SetLength handleSetLength#define IO_GetLength handleGetLength#ifdef _ALLBSD_SOURCE#define open64 open#define fstat64 fstat#define stat64 stat#define lseek64 lseek#define ftruncate64 ftruncate#define IO_Lseek lseek#else#define IO_Lseek lseek64#endif/* * On Solaris, the handle field is unused */#define SET_HANDLE(fd) return (jlong)-1/* * Retry the operation if it is interrupted */#define RESTARTABLE(_cmd, _result) do { \    do { \        _result = _cmd; \    } while((_result == -1) && (errno == EINTR)); \} while(0)/* * IO helper function(s) */void fileClose(JNIEnv *env, jobject this, jfieldID fid);#ifdef MACOSXjstring newStringPlatform(JNIEnv *env, const char* str);#endif

所以，fo.getFD().sync()方法本质上调用了系统的fsync方法。这个层面，java和c可以做到同样的控制粒度了。