【设计模式】单例模式的5种实现（饿汉式、懒汉式、双检锁、静态内部类、枚举）

饿汉式

/**
 * 饿汉式：直接创建好对象实例，调用直接返回，不能延迟加载
 */
public class Singleton {
    private static final Singleton singleton = new Singleton();

    private Singleton() {
    }

    public static Singleton getInstance() {
        return singleton;
    }
}

懒汉式

/**
 * 懒汉式：在获取实例时判断对象是否存在决定创建还是直接返回，因为有synchronized，并发高性能差
 */
public class Singleton2 {
    private static Singleton2 singleton;

    private Singleton2() {
    }

    // 对整个方法加锁
    public synchronized static Singleton2 getInstance() {
        if (singleton == null) {
            singleton = new Singleton2();
        }
        return singleton;
    }
}

懒汉式双检锁

/**
 * 懒汉式优化：双检锁，降低锁的粒度，只有实例是空才会加锁
 */
public class Singleton3 {
    private static volatile Singleton3 singleton;

    private Singleton3() {
    }

    public static Singleton3 getInstance() {
        if (singleton == null) {
            // 如果为空，获取锁
            synchronized (Singleton3.class) {
                // 如果依旧为空，创建对象
                if (singleton == null) {
                    singleton = new Singleton3();
                }
            }
        }
        return singleton;
    }
}

静态内部类

/**
 * 静态内部类：加载内部类的时候才会创建对象，可以延迟加载
 */
public class Singleton4 {

    private Singleton4() {
    }

    private static class SingletonInstance {
        private static final Singleton4 singleton = new Singleton4();
    }

    public static Singleton4 getInstance() {
        return SingletonInstance.singleton;
    }
}

枚举

/**
 * 枚举
 */
public enum Singleton5 {
    INSTANCE;

    public static Singleton5 getInstance() {
        return INSTANCE;
    }
}