多线程那些糟糕的代码——你这样干过吗？

来源：互联网发布：ip什么意思网络用语编辑：程序博客网时间：2024/05/17 04:40

最近在烦恼用java操作大文件的问题——如果我有一大堆数据在内存中，想要写入文件，如何提速？

我们都知道 Java 中的 RandomAccessFile 可以访问文件的任意位置，另外， java 中想要提速的方法有缓存，多线程操作等。而在这一环节中我首先想到的就是多线程写入文件，这样可以提升速度的吧？

但是我们知道，无论何时文件的写操作，底层都会加锁（这会影响性能就不解释了），为了验证这个锁对多线程的影响有多大，下面做了一个实验，首先准备要写入文件的字符内容（500个字符）：

int len = 500;            int dimen1 = 5;            int dimen2 = len / dimen1;            char data[] = new char[len];            char dd[][] = new char[dimen1][dimen2];            for (int i = 0; i < dimen1; i++) {                for (int j = 0; j < dimen2; j++) {                    dd[i][j] = (char) ('a' + i);                    data[i * dimen2 + j] = (char) ('a' + i);                }            }

为了能够得到一个比较好的结果，统计十次：

    for (int k = 0; k < 10; k++) {                CountDownLatch cl = new CountDownLatch(dd.length);                long start = System.currentTimeMillis();                for (int i = 0; i < dd.length; i++) {                    new Thread(new RwThread(new RandomAccessFile(f, "rws"),                            i * dimen2*2, i * dimen2*2 + dimen2*2, dd[i], cl)).start();                }                cl.await();                System.out.println("duration: "                        + (System.currentTimeMillis() - start));            }

多线程谢文件的代码如下：

import java.io.RandomAccessFile;import java.util.concurrent.CountDownLatch;public static class RwThread implements Runnable {    private RandomAccessFile raf;    private int start;    private int end;    private char[] data;    private CountDownLatch latch;    public RwThread(RandomAccessFile raf, int start, int end, char[] data,            CountDownLatch cl) {        this.raf = raf;        this.start = start;        this.end = end;        this.data = data;        latch = cl;    }    @Override    public void run() {        try {            raf.seek(start);            for (int i = 0; i < data.length; i++) {                raf.writeChar(data[i]);            }        } catch (Exception e) {        } finally {            try {                if (raf != null) {                    raf.close();                    raf = null;                }            } catch (Exception e) {            }        }        latch.countDown();    }}

得到耗时输出结果如下：

duration: 130duration: 114duration: 115duration: 127duration: 124duration: 117duration: 114duration: 119duration: 118duration: 119

首先，看到这个结果我就惊呆了，写 500 个字符尽然花费了 100多 ms ........

那他和单线程操作的差距有多大呢？请继续往下看：

同样统计十次，代码如下：

for (int k = 0; k < 10; k++) {                CountDownLatch cl = new CountDownLatch(1);                long start = System.currentTimeMillis();                new Thread(new RwThread(new RandomAccessFile(f, "rw"), 0,                        len * 2, data, cl)).start();                cl.await();                System.out.println("duration: "                        + (System.currentTimeMillis() - start));            }

得到耗时输出结果：

duration: 5duration: 2duration: 3duration: 2duration: 3duration: 2duration: 2duration: 2duration: 2duration: 2

天啦，结果居然相差了几十倍呢

今天的内容就到这里了，下面是完整的代码 ......

import java.io.File;import java.io.RandomAccessFile;import java.util.concurrent.CountDownLatch;/** * @author edwin *  */public class CreateTmpFile {    public static void main(String[] args) throws Exception {        String root = "F:" + File.separator + "tmpLog";        File f = new File(root, "ttt.txt");        File ddd = new File(root, "ttt1.txt");        if (!f.exists()) {            f.createNewFile();        }        if (!ddd.exists()) {            ddd.createNewFile();        }        System.out.println(f.getAbsolutePath());        int len = 500;        int dimen1 = 5;        int dimen2 = len / dimen1;        char data[] = new char[len];        char dd[][] = new char[dimen1][dimen2];        for (int i = 0; i < dimen1; i++) {            for (int j = 0; j < dimen2; j++) {                dd[i][j] = (char) ('a' + i);                data[i * dimen2 + j] = (char) ('a' + i);            }        }        for (int k = 0; k < 10; k++) {            CountDownLatch cl = new CountDownLatch(1);            long start = System.currentTimeMillis();            new Thread(new RwThread(new RandomAccessFile(f, "rw"), 0, len * 2,                    data, cl)).start();            cl.await();            System.out.println("duration: "                    + (System.currentTimeMillis() - start));        }        // for (int k = 0; k < 10; k++) {        // CountDownLatch cl = new CountDownLatch(dd.length);        // long start = System.currentTimeMillis();        // for (int i = 0; i < dd.length; i++) {        // new Thread(new RwThread(new RandomAccessFile(f, "rws"),        // i * dimen2*2, i * dimen2*2 + dimen2*2, dd[i], cl)).start();        // }        // cl.await();        // System.out.println("duration: "        // + (System.currentTimeMillis() - start));        // }    }    public static class RwThread implements Runnable {        private RandomAccessFile raf;        private int start;        private int end;        private char[] data;        private CountDownLatch latch;        public RwThread(RandomAccessFile raf, int start, int end, char[] data,                CountDownLatch cl) {            this.raf = raf;            this.start = start;            this.end = end;            this.data = data;            latch = cl;        }        @Override        public void run() {            try {                raf.seek(start);                for (int i = 0; i < data.length; i++) {                    raf.writeChar(data[i]);                }            } catch (Exception e) {            } finally {                try {                    if (raf != null) {                        raf.close();                        raf = null;                    }                } catch (Exception e) {                }            }            latch.countDown();        }    }}

0 0