在多线程环境下对字符串进行MD5，到底应该使用加锁来共享同一个MessageDigest呢？还是每次新创建一个，个人认为需要 根据程序运行的环境来分别对待。下边是从org.springframework.extensions.surf摘取的一段代码，实现了两种调用方式， 不过到底在何种情况下使用何种方式，目前还不是很清晰，希望通过测试能够得出结论。
Java代码
- import java.security.MessageDigest;
- import java.security.NoSuchAlgorithmException;
- /**
- * The MD5 utility class computes the MD5 digest (aka: "hash") of a block
- * of data; an MD5 digest is a 32-char ASCII string.
- *
- * The synchronized/static function "Digest" is useful for situations where
- * lock contention in the application is not expected to be an issue.
- *
- * The unsynchronized/non-static method "digest" is useful in a
- * multi-threaded program that wanted to avoid locking by creating
- * an MD5 object for exclusive use by a single thread.
- *
- *
- *
- * EXAMPLE 1: Static usage
- *
- * import org.springframework.extensions.surf.util.MD5;
- * String x = MD5.Digest("hello".getBytes());
- *
- *
- * EXAMPLE 2: Per-thread non-static usage
- *
- * import org.springframework.extensions.surf.util.MD5;
- * MD5 md5 = new MD5();
- * ...
- * String x = md5.digest("hello".getBytes());
- *
- *
- * Email: diwayou@163.com
- * User: diwayou
- * Date: 13-4-15
- * Time: 下午11:18
- */
- public class MD5 {
- private static final byte[] ToHex_ =
- { '0','1','2','3','4','5','6','7',
- '8','9','a','b','c','d','e','f'
- };
- private MessageDigest md5_ = null;
- static private MessageDigest Md5_;
- static
- {
- try { Md5_ = MessageDigest.getInstance("MD5");} // MD5 is supported
- catch ( NoSuchAlgorithmException e ) {}; // safe to swallow
- };
- /**
- * Constructor for use with the unsynchronized/non-static method
- * "digest" method. Note that the "digest" function is not
- * thread-safe, so if you want to use it, every thread must create
- * its own MD5 instance. If you don't want to bother & are willing
- * to deal with the potential for lock contention, use the synchronized
- * static "Digest" function instead of creating an instance via this
- * constructor.
- */
- public MD5()
- {
- try { md5_ = MessageDigest.getInstance("MD5");} // MD5 is supported
- catch ( NoSuchAlgorithmException e ) {}; // safe to swallow
- }
- /**
- * Thread-safe static digest (hashing) function.
- *
- * If you want to avoid lock contention, create an instance of MD5
- * per-thead, anc call the unsynchronized method 'digest' instead.
- */
- public static synchronized String Digest(byte[] dataToHash)
- {
- Md5_.update(dataToHash, 0, dataToHash.length);
- return HexStringFromBytes( Md5_.digest() );
- }
- /**
- * Non-threadsafe MD5 digest (hashing) function
- */
- public String digest(byte[] dataToHash)
- {
- md5_.update(dataToHash, 0, dataToHash.length);
- return HexStringFromBytes( md5_.digest() );
- }
- private static String HexStringFromBytes(byte[] b)
- {
- byte [] hex_bytes = new byte[ b.length * 2 ];
- int i,j=0;
- for (i=0; i < b.length; i++)
- {
- hex_bytes[j] = ToHex_[ ( b[i] & 0x000000F0 ) >> 4 ] ;
- hex_bytes[j+1] = ToHex_[ b[i] & 0x0000000F ];
- j+=2;
- }
- return new String( hex_bytes );
- }
- }
下边是闲暇时写的一段测试代码，测试方式考虑因素还是比较少的，只考虑到了竞争线程的数量，代码如下：
Java代码
- package com.diwayou.logq;
- import com.diwayou.logq.util.MD5;
- import org.jfree.chart.ChartFactory;
- import org.jfree.chart.ChartFrame;
- import org.jfree.chart.JFreeChart;
- import org.jfree.chart.plot.PlotOrientation;
- import org.jfree.data.xy.DefaultXYDataset;
- import org.jfree.data.xy.XYDataset;
- import java.io.UnsupportedEncodingException;
- import java.util.concurrent.ExecutorService;
- import java.util.concurrent.Executors;
- /**
- * Email: diwayou@163.com
- * User: diwayou
- * Date: 13-3-26
- * Time: 下午5:48
- */
- public class LogQ {
- public static void main(String[] args) throws UnsupportedEncodingException, InterruptedException {
- String s = "alibabagogogo";
- final byte[] message = s.getBytes("GBK");
- int coreNum = Runtime.getRuntime().availableProcessors();
- int scale = 100;
- System.out.println(String.format("Core size is %d", coreNum));
- int TEST_TIMES = 1;
- long startTime, endTime;
- DefaultXYDataset xyDataset = new DefaultXYDataset();
- double[][] elapse = new double[2][scale];
- for (int j = 0; j < scale; j++) {
- ExecutorService executorService = Executors.newFixedThreadPool(scale);
- startTime = System.nanoTime();
- for (int i = 0; i < TEST_TIMES; i++) {
- executorService.submit(new Runnable() {
- @Override
- public void run() {
- MD5.Digest(message);
- }
- });
- }
- executorService.shutdown();
- endTime = System.nanoTime();
- System.out.println(String.format("Pool size %d, elapse time %d", j, (endTime - startTime)));
- elapse[0][j] = j;
- if (j == 0) {
- elapse[1][j] = 0;
- } else {
- elapse[1][j] = endTime - startTime;
- }
- }
- xyDataset.addSeries("Synchronized", elapse);
- elapse = new double[2][scale];
- for (int j = 0; j < scale; j++) {
- ExecutorService executorService = Executors.newFixedThreadPool(scale);
- startTime = System.nanoTime();
- for (int i = 0; i < TEST_TIMES; i++) {
- executorService.submit(new Runnable() {
- @Override
- public void run() {
- MD5 md5 = new MD5();
- md5.digest(message);
- }
- });
- }
- executorService.shutdown();
- endTime = System.nanoTime();
- System.out.println(String.format("Pool size %d, elapse time %d", j, (endTime - startTime)));
- elapse[0][j] = j;
- if (j == 0) {
- elapse[1][j] = 0;
- } else {
- elapse[1][j] = endTime - startTime;
- }
- }
- xyDataset.addSeries("NewEveryTime", elapse);
- displayChart("Synchronized VS NewEveryTime", xyDataset);
- }
- private static void displayChart(String title, XYDataset dataset) {
- JFreeChart xyLineChart = ChartFactory.createXYLineChart(title, "Pool Size", "Elapse Time", dataset,
- PlotOrientation.VERTICAL, true, true, true);
- ChartFrame chartFrame = new ChartFrame("Stat Result", xyLineChart);
- chartFrame.pack();
- chartFrame.setVisible(true);
- }
- }
运行结果如下：
（1）第一次
Core size is 8Pool size 0, elapse time 2012874Pool size 1, elapse time 839776..............（2）第二次
Core size is 8Pool size 0, elapse time 2256508Pool size 1, elapse time 795155Pool size 2, elapse time 968285...................结论：通过测试结果，可以看出每次都new一个新的并不比共享同一个MessageDigest慢，而且不需要锁，这样在服务器高并发的环境下，就不会出现共享锁性能瓶颈的问题，这样可以减少由于共享锁出现的上下文切换，个人倾向于每次都new一个。本人菜鸟一个，分析有误还请大家支出，随便批评，这样我才能进步。