单例模式怎样写

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class StringManager {

private static StringManager instance;

private Map beanMap;

private StringManager() {

beanMap = new HashMap();

}

public static StringManager getInstance() {

if (instance == null) {

instance = new StringManager();

}

return instance;

}

public Object getBean(String beanName) {

return beanMap.get(beanName);

}

编写一个单例模式程序

懒汉式单例模式是延迟加载的单例设计模式

public class lazysingleton {

//静态私有的成员变量

private static lazysingleton instance = null;

// 私有的构造方法

private lazysingleton() {}

// 如果不加synchronized会导致对线程的访问不安全

// 双重锁定检查

public static lazysingleton getinstance() {

if (instance == null) {

synchronized (lazysingleton.class) {

if (null == instance) {

instance = new lazysingleton();

}

return instance;

}

单例模式代码怎么写

单例模式（Singleton），属于最常见的设计模式之一，大部分系统都会用到，目的是为了维护系统中唯一的一个实例。可分为eager模式，示例代码如下： Java代码 1.class EagerSingleton{ 2. private static final EagerSingleton m_instance = new EagerSingleton(); 3. private EagerSingleton(){} 4. public static EagerSingleton getInstance(){ 5. return m_instance; 6. } 7.} class EagerSingleton{ private static final EagerSingleton m_instance = new EagerSingleton(); private EagerSingleton(){} public static EagerSingleton getInstance(){ return m_instance； } } 和 lazy模式，示例代码如下： Java代码 1.class LazySingleton{ 2. private static LazySingleton m_instance = null; 3. private LazySingleton(){} 4. public synchronized static getInstance(){ 5. if(m_instance == null){ 6. m_instance = new LazySingleton(); 7. } 8. return m_instance; 9. } 10.} class LazySingleton{ private static LazySingleton m_instance = null; private LazySingleton(){} public synchronized static getInstance(){ if(m_instance == null){ m_instance = new LazySingleton(); } return m_instance; } } java源码中，Runtime.getRuntime（)就是单例的一个例子。

单例模式的精神就是整个系统中维护一个实例，推广开来，如果在一个系统中需要维护多个示例，那么就产生了多例模式（multiton）。多例模式（Multiton），通过聚集对象了保留自身的多个示例，根据客户端的参数返回所需要的实例。示例代码如下： Java代码 1.class Multiton{ 2. private final int INSTANCE_SIZE = 10; 3. private static Map instances = new HashMap(INSTANCE_SIZE); 4. private String name; 5. private Multiton(){} 6. private Multiton(String name){ 7. this.name = name; 8. } 9. public synchronized static getInstance(String name){ 10. if(instances.containsKey(name)){ 11. return instances.get(name); 12. } 13. else{ 14. ins = new Multiton(name); 15. instances.put(name, ins); 16. return ins; 17. } 18. } 19.} class Multiton{ private final int INSTANCE_SIZE = 10; private static Map instances = new HashMap(INSTANCE_SIZE); private String name; private Multiton(){} private Multiton(String name){ this.name = name; } public synchronized static getInstance(String name){ if(instances.containsKey(name)){ return instances.get(name); } else{ ins = new Multiton(name); instances.put(name, ins); return ins; } } } [nextpage] 一个实用的例子就是KeyGenerator，示例代码如下： Java代码 1.class KeyGenerator{ 2. private final int POOL_SIZE = 20; 3. private static Map instances = new HashMap(16); 4. private KeyInfo keyinfo; 5. private KeyGenerator(){} 6. private KeyGenerator(String keyName){ 7. this.keyinfo = new KeyInfo(POOL_SIZE, keyName); 8. } 9. public synchronized static getInstance(String keyName){ 10. if(instances.containsKey(keyName)){ 11. return (KeyGenerator)instances.get(keyName); 12. } 13. else{ 14. keyGen = new KeyGenerator(keyName); 15. instances.put(name, keyGen); 16. return keyGen; 17. } 18. } 19. public synzhronized int getNextKey(){ 20. return keyinfo.getNextKey(); 21. } 22. } class KeyGenerator{ private final int POOL_SIZE = 20; private static Map instances = new HashMap(16); private KeyInfo keyinfo; private KeyGenerator(){} private KeyGenerator(String keyName){ this.keyinfo = new KeyInfo(POOL_SIZE, keyName); } public synchronized static getInstance(String keyName){ if(instances.containsKey(keyName)){ return (KeyGenerator)instances.get(keyName); } else{ keyGen = new KeyGenerator(keyName); instances.put(name, keyGen); return keyGen; } } public synzhronized int getNextKey(){ return keyinfo.getNextKey(); } }