Java代码 
privatestaticClass getClass(String classname)
throwsClassNotFoundException {
ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
if(classLoader ==null)
classLoader = Singleton.class.getClassLoader();
return(classLoader.loadClass(classname));
}
}
private static Class getClass(String classname)
throws ClassNotFoundException {
ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
if(classLoader == null)
classLoader = Singleton.class.getClassLoader();
return (classLoader.loadClass(classname));
}
}
这个方法会尝试把当前的线程与那个类载入器相关联;如果classloader为null，这个方法会使用与装入单例类基类的那个类载入器。这个方法可以用Class.forName()代替。
序列化
如果你序列化一个单例类，然后两次重构它，那么你就会得到那个单例类的两个实例，除非你实现readResolve()方法，像下面这样：
例12 一个可序列化的单例类
Java代码
importorg.apache.log4j.Logger;
publicclassSingletonimplementsjava.io.Serializable {
publicstaticSingleton INSTANCE =newSingleton();
protectedSingleton() {
// Exists only to thwart instantiation.
}
privateObject readResolve() {
returnINSTANCE;
}
}
import org.apache.log4j.Logger;
public class Singleton implements java.io.Serializable {
public static Singleton INSTANCE = new Singleton();
protected Singleton() {
// Exists only to thwart instantiation.
}
private Object readResolve() {
return INSTANCE;
}
}
上面的单例类实现从readResolve()方法中返回一个唯一的实例;这样无论Singleton类何时被重构，它都只会返回那个相同的单例类实例。
例13测试了例12的单例类：
例13 测试一个可序列化的单例类
Java代码
importjava.io.*;
importorg.apache.log4j.Logger;
importjunit.framework.Assert;
importjunit.framework.TestCase;
publicclassSingletonTestextendsTestCase {
privateSingleton sone =null, stwo =null;
privatestaticLogger logger = Logger.getRootLogger();
publicSingletonTest(String name) {
super(name);
}
publicvoidsetUp() {
sone = Singleton.INSTANCE;
stwo = Singleton.INSTANCE;
}
publicvoidtestSerialize() {
logger.info("testing singleton serialization...");
writeSingleton();
Singleton s1 = readSingleton();
Singleton s2 = readSingleton();
Assert.assertEquals(true, s1 == s2); }
privatevoidwriteSingleton() {
try{
FileOutputStream fos =newFileOutputStream("serializedSingleton");
ObjectOutputStream oos =newObjectOutputStream(fos);
Singleton s = Singleton.INSTANCE;
oos.writeObject(Singleton.INSTANCE);
oos.flush();
}
catch(NotSerializableException se) {
logger.fatal("Not Serializable Exception: "+ se.getMessage());
}
catch(IOException iox) {
logger.fatal("IO Exception: "+ iox.getMessage());
}
}
privateSingleton readSingleton() {
Singleton s =null;
try{
FileInputStream fis =newFileInputStream("serializedSingleton");
ObjectInputStream ois =newObjectInputStream(fis);
s = (Singleton)ois.readObject();
}
catch(ClassNotFoundException cnf) {
logger.fatal("Class Not Found Exception: "+ cnf.getMessage());
}
catch(NotSerializableException se) {
logger.fatal("Not Serializable Exception: "+ se.getMessage());
}
catch(IOException iox) {
logger.fatal("IO Exception: "+ iox.getMessage());
}
returns;
}
publicvoidtestUnique() {
logger.info("testing singleton uniqueness...");
Singleton another =newSingleton();
logger.info("checking singletons for equality");
Assert.assertEquals(true, sone == stwo);
}
}
import java.io.*;
import org.apache.log4j.Logger;
import junit.framework.Assert;
import junit.framework.TestCase;
public class SingletonTest extends TestCase {
private Singleton sone = null, stwo = null;
private static Logger logger = Logger.getRootLogger();
public SingletonTest(String name) {
super(name);
}
public void setUp() {
sone = Singleton.INSTANCE;
stwo = Singleton.INSTANCE;
}
public void testSerialize() {
logger.info("testing singleton serialization...");
writeSingleton();
Singleton s1 = readSingleton();
Singleton s2 = readSingleton();
Assert.assertEquals(true, s1 == s2); }
private void writeSingleton() {
try {
FileOutputStream fos = new FileOutputStream("serializedSingleton");
ObjectOutputStream oos = new ObjectOutputStream(fos);
Singleton s = Singleton.INSTANCE;
oos.writeObject(Singleton.INSTANCE);
oos.flush();
}
catch(NotSerializableException se) {
logger.fatal("Not Serializable Exception: " + se.getMessage());
}
catch(IOException iox) {
logger.fatal("IO Exception: " + iox.getMessage());
}
}
private Singleton readSingleton() {
Singleton s = null;
try {
FileInputStream fis = new FileInputStream("serializedSingleton");
ObjectInputStream ois = new ObjectInputStream(fis);
s = (Singleton)ois.readObject();
}
catch(ClassNotFoundException cnf) {
logger.fatal("Class Not Found Exception: " + cnf.getMessage());
}
catch(NotSerializableException se) {
logger.fatal("Not Serializable Exception: " + se.getMessage());
}
catch(IOException iox) {
logger.fatal("IO Exception: " + iox.getMessage());
}
return s;
}
public void testUnique() {
logger.info("testing singleton uniqueness...");
Singleton another = new Singleton();
logger.info("checking singletons for equality");
Assert.assertEquals(true, sone == stwo);
}
}
前面这个测试案例序列化例12中的单例类，并且两次重构它。然后这个测试案例检查看是否被重构的单例类实例是同一个对象。下面是测试案例的输出：
Java代码
Buildfile: build.xml
init:
[echo] Build20030422(22-04-200311:32)
compile:
run-test-text:
[java] .INFO main: testing singleton serialization...
[java] .INFO main: testing singleton uniqueness...
[java] INFO main: checking singletonsforequality
[java] Time:0.1
[java] OK (2tests)
Buildfile: build.xml
init:
[echo] Build 20030422 (22-04-2003 11:32)
compile:
run-test-text:
[java] .INFO main: testing singleton serialization...
[java] .INFO main: testing singleton uniqueness...
[java] INFO main: checking singletons for equality
[java] Time: 0.1
[java] OK (2 tests)
单例模式结束语
单例模式简单却容易让人迷惑，特别是对于Java的开发者来说。在这篇文章中，作者演示了Java开发者在顾及多线程、类载入器和序列化情况如何实现单例模式。作者也展示了你怎样才能实现一个单例类的注册表，以便能够在运行期指定单例类。
posted on 2008-05-05 22:12 bcterry 阅读(38) 评论(0) 编辑 收藏
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