java MD5 多线程环境下共享锁还是每次创建一个新的MessageDigest

    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。

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: [e-mail]
 * 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：

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: [e-mail]
 * 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 8
Pool size 0, elapse time 2012874
Pool size 1, elapse time 839776
..............
（2）第二次

 
Core size is 8
Pool size 0, elapse time 2256508
Pool size 1, elapse time 795155
Pool 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。