（Session机制原理，如果有疑问，可查找相关文献。有个博客写的也挺实用的）
本人实用的Tomcat版本为5.0.28，以下内容都是针对此版本。
产生sessionId的类是，org.apache.catalina.session.StandardManagerextends ManagerBase，，代码实现就在ManagerBase.generateSessionId。
org.apache.catalina.session.ManagerBase是一个抽象类，为了方便直接进入主题，我对该类对了一些删减，只保留了方法generateSessionId相关的东东。
下面的就是我裁剪后的代码。
/* * Copyright 1999,2004 The Apache Software Foundation. * * Licensed under the Apache License, Version 2.0 (the "License"); * you may not use this file except in compliance with the License. * You may obtain a copy of the License at * * * * Unless required by applicable law or agreed to in writing, software * distributed under the License is distributed on an "AS IS" BASIS, * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. * See the License for the specific language governing permissions and * limitations under the License. */package org.apache.catalina.session;import java.security.MessageDigest;import java.security.NoSuchAlgorithmException;import java.util.Random;/** * Minimal implementation of the <b>Manager</b> interface that supports * no session persistence or distributable capabilities. This class may * be subclassed to create more sophisticated Manager implementations. * * @author Craig R. McClanahan * @version $Revision: 1.27 $ $Date: 2004/05/26 16:13:59 $ */public class ManagerBaseTest {// —————————————————– Instance Variables/*** The default message digest algorithm to use if we cannot use* the requested one.*/private static final String DEFAULT_ALGORITHM = "MD5";/*** The message digest algorithm to be used when generating session* identifiers. This must be an algorithm supported by the* <code>java.security.MessageDigest</code> class on your platform.*/private String algorithm = DEFAULT_ALGORITHM;/*** Return the MessageDigest implementation to be used when* creating session identifiers.*/private MessageDigest digest = null;/*** A String initialization parameter used to increase the entropy of* the initialization of our random number generator.*/private String entropy = null;/*** The session id length of Sessions created by this Manager.*/private int sessionIdLength = 16;/*** A random number generator to use when generating session identifiers.*/private Random random = null;/*** The Java class name of the random number generator class to be used* when generating session identifiers.*/private String randomClass = "java.security.SecureRandom";// ————————————————————- Properties/*** Return the entropy increaser value, or compute a semi-useful value* if this String has not yet been set.*/public String getEntropy() {// Calculate a semi-useful value if this has not been setif (this.entropy == null)setEntropy(this.toString());return (this.entropy);}/*** Set the entropy increaser value.** @param entropy The new entropy increaser value*/public void setEntropy(String entropy) {this.entropy = entropy;}/*** Return the MessageDigest object to be used for calculating* session identifiers. If none has been created yet, initialize* one the first time this method is called.*/public synchronized MessageDigest getDigest() {if (this.digest == null) {long t1=System.currentTimeMillis();try {this.digest = MessageDigest.getInstance(algorithm);} catch (NoSuchAlgorithmException e) {try {this.digest = MessageDigest.getInstance(DEFAULT_ALGORITHM);} catch (NoSuchAlgorithmException f) {this.digest = null;}}long t2=System.currentTimeMillis();}return (this.digest);}/*** Return the random number generator instance we should use for* generating session identifiers. If there is no such generator* currently defined, construct and seed a new one.*/public synchronized Random getRandom() {if (this.random == null) {synchronized (this) {if (this.random == null) {// Calculate the new random number generator seedlong seed = System.currentTimeMillis();long t1 = seed;char entropy[] = getEntropy().toCharArray();for (int i = 0; i < entropy.length; i++) {long update = ((byte) entropy[i]) << ((i % 8) * 8);seed ^= update;}try {// Construct and seed a new random number generatorClass clazz = Class.forName(randomClass);this.random = (Random) clazz.newInstance();this.random.setSeed(seed);} catch (Exception e) {// Fall back to the simple casethis.random = new java.util.Random();this.random.setSeed(seed);}long t2=System.currentTimeMillis();}}}return (this.random);}protected void getRandomBytes( byte bytes[] ) {Random random = getRandom();getRandom().nextBytes(bytes);}/*** Generate and return a new session identifier.*/public synchronized String generateSessionId() {byte random[] = new byte[16];// Render the result as a String of hexadecimal digitsStringBuffer result = new StringBuffer();int resultLenBytes = 0;while (resultLenBytes < this.sessionIdLength) {getRandomBytes(random);random = getDigest().digest(random);for (int j = 0;j < random.length && resultLenBytes < this.sessionIdLength;j++) {byte b1 = (byte) ((random[j] & 0xf0) >> 4);byte b2 = (byte) (random[j] & 0x0f);if (b1 < 10)result.append((char) ('0' + b1));elseresult.append((char) ('A' + (b1 – 10)));if (b2 < 10)result.append((char) ('0' + b2));elseresult.append((char) ('A' + (b2 – 10)));resultLenBytes++;}}return (result.toString());}/** * @Description: * @param args * @author mahh * @since：2015-5-20 下午02:07:45 */public static void main(String[] args) {ManagerBaseTest managerBaseTest = new ManagerBaseTest();System.out.println(managerBaseTest.generateSessionId());}}
看了哪些风景，遇到哪些人。尽管同学说，去旅行不在于记忆，
