1.懒汉模式1.构建单例对象
ps:为了测试再一次创建的Singleton对象是否是同一对象,为其添加属性name
public class Singleton { //添加属性name private String name; private static Singleton instance; //私有的构造方法,使其不能通过new得到该对象 private Singleton() {} public static Singleton getInstance() { if (instance == null) {//判断是否为空,实现单例 instance = new Singleton(); } return instance; } //get和set方法 public String getName() { return name; } public void setName(String name) { this.name = name; } @Override public String toString() { return "Singleton [name=" + name + "]"; }}
2.测试
public class HungryModeTest { @Test public void hungryMode(){ //通过其方法创建singleton对象 Singleton singleton = Singleton.getInstance(); //为name属性赋值 singleton.setName("张三"); System.err.println(singleton.toString()); //创建新的singleton对象singletonNew Singleton singletonNew= Singleton.getInstance(); System.err.println(singletonNew.toString()); }}
3.测试输出结果
4.总结:
懒汉式当需要的时候才会创建对象,但是当多个线程同时加载时,懒汉就会出现同时构造了多个对象的问题,懒汉式是线程不安全的.
2.线程安全的懒汉模式1.为普通懒汉试添加线程锁
ps:添加红色部分为线程锁,解决多线程的问题
public class Singleton { //添加属性name private String name; private static Singleton instance; //私有的构造方法,使其不能通过new得到该对象 private Singleton() {} public static synchronized Singleton getInstance() { if (instance == null) {//判断是否为空,实现单例 instance = new Singleton(); } return instance; } //get和set方法 public String getName() { return name; } public void setName(String name) { this.name = name; } @Override public String toString() { return "Singleton [name=" + name + "]"; }}
3.饿汉式1.创建单例对象
public class Singleton { // 添加属性name private String name; private static Singleton instance = new Singleton(); //私有的构造方法 private Singleton() {} public static Singleton getInstance() { return instance; } // get和set方法 public String getName() {return name;} public void setName(String name) {this.name = name;} public String toString() {return "Singleton [name=" + name + "]";}}
2.测试
ps:测试代码与上面懒汉测试代码相同
3.测试结果
4.双重校验锁1.加锁的懒汉模会出现延迟加载,而双重校验锁是更进一步的改进
public class Singleton { // 添加属性name private String name; //通过volatile禁止指令重排序优化,保证对象初始化 private static volatile Singleton instance = null; private Singleton() {} public static Singleton getInstance() { if (instance == null) {//为空判断保证单例 synchronized (Singleton.class) {//线程锁 if (instance == null) {//在一次为空判断,保证多线程 instance = new Singleton(); } } } return instance; } // get和set方法 public String getName() { return name; } public void setName(String name) { this.name = name; } public String toString() { return "Singleton [name=" + name + "]"; }}
5.静态内部类1.利用类加载机制来保证只创建一个实例
public class Singleton { // 添加属性name private String name; private static class SingletonInner{ //内部类 public static Singleton instance = new Singleton(); } private Singleton(){} //私有构造 public static Singleton newInstance(){ return SingletonInner.instance; } // get和set方法 public String getName() { return name; } public void setName(String name) { this.name = name; } public String toString() { return "Singleton [name=" + name + "]"; }}
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