二十一、单例模式

设计一个类，只能产生该类的一个实例。

方法一：懒汉式，线程不安全

package com.pattern.singleton;

public class Singleton1 {
//  private static boolean isCreated = false;

    private static Singleton1 instance = null;

    private Singleton1() {}

    public static Singleton1 getInstance() {
        if(instance == null) {
            instance = new Singleton1();
        } 
        return instance;
    }
}

上述代码只能确保在单线程情况下满足要求，在多线程方式下可能出错！

方法二：懒汉式，线程安全，多线程环境下效率不高

使用synchronized关键字进行同步：

public synchronized static Singleton1 getInstance() {
        if(instance == null) {
            instance = new Singleton1();
        } 
        return instance;
    }

package com.pattern.singleton;

public class Singleton1 {
//  private static boolean isCreated = false;
    private static Object syncObj = new Object();
    private static Singleton1 instance = null;

    private Singleton1() {}

    public static Singleton1 getInstance() {
        synchronized(syncObj) {
            if(instance == null) {
                instance = new Singleton1();
            } 
        }
        return instance;
    }
}

我们只需要在还没有创建实例的时候才会进行同步锁，以保证只有一个线程去创建实例。而当实例已经创建成果后，就没必要进行加锁的操作了。比上述方法效率更高，改进后的代码如下：

方法三：使用双重校验锁，线程安全【推荐】

package com.pattern.singleton;

public class Singleton1 {
//  private static boolean isCreated = false;
    private static Object syncObj = new Object();
    private static Singleton1 instance = null;

    private Singleton1() {}

    public static Singleton1 getInstance() {
        if(instance == null) {
            synchronized(syncObj) {
                if(instance == null) {
                    instance = new Singleton1();
                } 
            }
        }
        return instance;
    }
}

方法四：饿汉式，线程安全

package com.pattern.singleton;

public class Singleton1 {
    private static Singleton1 instance = new Singleton1();

//  static {
//      instance = new Singleton1();
//  }

    private Singleton1() {}

    public static Singleton1 getInstance() {
        return instance;
    }
}

该方法存在一个问题：就是会有过早创建实例的问题！降低了内存的使用效率！

方法五：使用静态内部类，线程安全【推荐】

package com.pattern.singleton;

public class Singleton1 {
    private Singleton1() {}

    public static Singleton1 getInstance() {
        return Nested.instance;
    }

    private static class Nested {
        private Nested() {}
        private static final Singleton1 instance = new Singleton1(); 
    }
}