本文给出了一个如何利用java提供的类MappedByteBuffer对文件进行并发写入的例子。具体实现思路就举个列子说明吧： 假设有10000个字节需要写入某个文件，为了加快写入速度，可以开启2个线程，第一个线程将前5000个字节写入文件0~4999的位置，另外一个线程将后5000个字节写入文件5000~9999的位置，等2个线程都写入完成后。我们就成功的将10000个字节完整的写入到了文件中。
值得一提的是，线程在写入过程中是会对自己所占用的那个文件区域枷锁的。各个线程加锁的区域互不重叠，否则会报OverlappingFileLockException异常
这个例子读者可以直接运行main方法查看效果，该类无任何外部依赖，全是jdk自带api。待写入的内容用源码中CONTENS字段表示，该字段记录了欧阳修名作《秋声赋》的完整内容。程序运行结束后可以到D盘查找名为ConcurrentWrite.txt的文件查看写入结果（D://ConcurrentWrite.txt）,当然了，前提是你的电脑存在D盘。
程序不是很复杂，直接贴源码吧，关键地方有注释。另外，目标文件ConcurrentWrite.txt在程序运行期间不能被其他外部程序打开，否则写入失败！！！！
package com.lhever.modules.io.test;import java.io.*;import java.lang.reflect.Method;import java.nio.MappedByteBuffer;import java.nio.channels.FileChannel;import java.nio.channels.FileLock;import java.nio.channels.OverlappingFileLockException;import java.security.AccessController;import java.security.PrivilegedAction;import java.util.ArrayList;import java.util.List;import java.util.concurrent.CountDownLatch;public class MappedByteBufferTest {//该字符串的内容将会被多线程写入到文件中static String CONTENS = "【作者】欧阳修 【朝代】宋 译文对照\n" +"欧阳子方夜读书，闻有声自西南来者，悚然而听之，曰：“异哉！”初淅沥以萧飒，忽奔腾而砰湃，如波涛夜惊，风雨骤至。其触于物也，鏦鏦铮铮，金铁皆鸣；又如赴敌之兵，衔枚疾走，不闻号令，但闻人马之行声。予谓童子：“此何声也？汝出视之。”童子曰：“星月皎洁，明河在天，四无人声，声在树间。”\n" +"\n" +"予曰：“噫嘻悲哉！此秋声也，胡为而来哉？盖夫秋之为状也：其色惨淡，烟霏云敛；其容清明，天高日晶；其气栗冽，砭人肌骨；其意萧条，山川寂寥。故其为声也，凄凄切切，呼号愤发。丰草绿缛而争茂，佳木葱茏而可悦；草拂之而色变，木遭之而叶脱。其所以摧败零落者，乃其一气之余烈。夫秋，刑官也，于时为阴；又兵象也，于行用金，是谓天地之义气，常以肃杀而为心。天之于物，春生秋实，故其在乐也，商声主西方之音，夷则为七月之律。商，伤也，物既老而悲伤；夷，戮也，物过盛而当杀。”\n" +"\n" +"“嗟乎！草木无情，有时飘零。人为动物，惟物之灵；百忧感其心，万事劳其形；有动于中，必摇其精。而况思其力之所不及，忧其智之所不能；宜其渥然丹者为槁木，黟然黑者为星星。奈何以非金石之质，欲与草木而争荣？念谁为之戕贼，亦何恨乎秋声！”\n" +"\n" +"童子莫对，垂头而睡。但闻四壁虫声唧唧，如助予之叹息耶。";public static void await(CountDownLatch latch) {if (latch == null) {return;}try {latch.await();} catch (InterruptedException e) {e.printStackTrace();}}public static void main(String... args) {File file = getByName("D://ConcurrentWrite.txt");byte[] bytes = null;try {bytes = CONTENS.getBytes("UTF-8");} catch (UnsupportedEncodingException e) {e.printStackTrace();}int len = bytes.length;System.out.println("len: " + len);int concurrentNum = 4;if (len < concurrentNum) {concurrentNum = 1;} else {concurrentNum = (len % concurrentNum == 0) ? concurrentNum : (concurrentNum + 1);}int size = len / concurrentNum;List<FileBlockWriter> writers = new ArrayList<FileBlockWriter>();CountDownLatch startLatch = new CountDownLatch(1);CountDownLatch endLatch = new CountDownLatch(concurrentNum);for (int i = 0; i < concurrentNum; i++) {FileBlockWriter writer = null;int from = i * size;int sliceLen = 0;System.out.println(from);if (i == concurrentNum - 1) {sliceLen = bytes.length - i * size;System.out.println(sliceLen);} else {sliceLen = size;}byte[] slice = new byte[sliceLen];System.arraycopy(bytes, from, slice, 0, sliceLen);writer = new FileBlockWriter(file, from, sliceLen, slice, startLatch, endLatch);writers.add(writer);}for (FileBlockWriter writer : writers) {writer.start();}//调用countDown后，所有写入线程才真正开始写入工作startLatch.countDown();//等待所有线程写入完成await(endLatch);System.out.println("done !!! ");}private static File getByName(String path) {File file = new File(path);if (!file.exists()) {try {file.createNewFile();} catch (IOException e) {throw new IllegalArgumentException("create file failed", e);}}if (file.isDirectory()) {throw new IllegalArgumentException("not a file");}return file;}/*** 该线程类负责将指定的内容(contents)写入文件（target）,* 写入的起始位置是：from，往后写的字节数目是：length*/public static class FileBlockWriter extends Thread {private File target;private int from;private int length;private byte[] contents;private CountDownLatch start;private CountDownLatch end;public FileBlockWriter(File target, int from, int length, byte[] contents, CountDownLatch start, CountDownLatch end) {this.target = target;this.from = from;this.length = length;this.contents = contents;this.start = start;this.end = end;}@Overridepublic void run() {RandomAccessFile randFile = null;FileChannel channel = null;MappedByteBuffer mbb = null;FileLock fileLock = null;MappedByteBufferTest.await(start);try {randFile = new RandomAccessFile(target, "rw");channel = randFile.getChannel();mbb = channel.map(FileChannel.MapMode.READ_WRITE, from, length);fileLock = channel.lock(from, length, true);while (fileLock == null || !fileLock.isValid()) {fileLock = channel.lock(from, length, true);System.out.print("锁无效，重复获取");}mbb.put(contents);mbb.force();} catch (IOException e) {e.printStackTrace();} catch (OverlappingFileLockException e) {e.printStackTrace();throw new IllegalArgumentException("程序设计不合理，加锁区域相互重叠");} catch (Exception e) {e.printStackTrace();} finally {release(fileLock);forceClose(mbb);close(channel, randFile);}end.countDown();}}private static void release(FileLock fileLock) {if (fileLock == null) {return;}try {fileLock.release();} catch (IOException e) {e.printStackTrace();}}private static void close(Closeable... closeables) {if (closeables == null || closeables.length == 0) {return;}for (Closeable closeable : closeables) {if (closeable == null) {continue;}try {closeable.close();} catch (IOException e) {e.printStackTrace();}}}/*** 强制关闭MappedByteBuffer* @param mbb*/private static void forceClose(MappedByteBuffer mbb) {AccessController.doPrivileged(new PrivilegedAction() {public Object run() {try {Method getCleanerMethod = mbb.getClass().getMethod("cleaner", new Class[0]);getCleanerMethod.setAccessible(true);sun.misc.Cleaner cleaner = (sun.misc.Cleaner)getCleanerMethod.invoke(mbb, new Object[0]);cleaner.clean();} catch (Exception e) {e.printStackTrace();}return null;}});}}
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