使用多线程往同一个文件写入数据的线程安全的例子(java实现)
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2024-03-21 20:12:40
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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;
}
@Override
public 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;
}
});
}
}
879675643@qq.com lhever
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