Java单例模式
单例类只能有一个实例
单例类必须自己创建自己的唯一实例
单例类必须给其他所有对象提供这一实例
实现：
1.懒汉式
public class Singleton {
private static Singleton instance;
private Singleton (){}
public static synchronized Singleton getInstance() {
if (instance == null) {
instance = new Singleton();
}
return instance;
}
} 2. 饿汉式
public class Singleton {
private static Singleton instance = new Singleton();
private Singleton (){}
public static Singleton getInstance() {
return instance;
}
} 3. 双重校验锁
public class Singleton {
private volatile static Singleton singleton;
private Singleton (){}
public static Singleton getSingleton() {
if (singleton == null) {
synchronized (Singleton.class) {
if (singleton == null) {
singleton = new Singleton();
}
}
}
return singleton;
}
} 
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