设计模式

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单例模式 ​

保证一个类仅有一个实例，并提供一个访问它的全局访问点。

JDK中的使用 ​

  • java.lang.Runtime#getRuntime()
  • java.awt.Desktop#getDesktop()
  • java.lang.System#getSecurityManager()

代码实现 ​

懒汉式 ​

实例在第一次使用的时候初始化

java
public class LazySingleton {
    private static LazySingleton instance;

    private LazySingleton() {

    }

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

        return instance;
    }
}
public class LazySingleton {
    private static LazySingleton instance;

    private LazySingleton() {

    }

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

        return instance;
    }
}

双重锁实现 ​

懒汉式在多线程情况下会失效 由此进化而来

java
public class DoubleLockSingleton {
    private static volatile DoubleLockSingleton instance;

    private DoubleLockSingleton() {

    }

    public static DoubleLockSingleton getInstance() {
        if (null == instance) {
            synchronized (DoubleLockSingleton.class) {
                if (null == instance) {
                    instance = new DoubleLockSingleton();
                }
            }
        }

        return instance;
    }
}
public class DoubleLockSingleton {
    private static volatile DoubleLockSingleton instance;

    private DoubleLockSingleton() {

    }

    public static DoubleLockSingleton getInstance() {
        if (null == instance) {
            synchronized (DoubleLockSingleton.class) {
                if (null == instance) {
                    instance = new DoubleLockSingleton();
                }
            }
        }

        return instance;
    }
}

饿汉式 ​

实例在类加载的时候初始化

java
public class HungrySingleton {
    private HungrySingleton() {

    }

    public static HungrySingleton getInstance() {
        return SingletonHolder.INSTANCE;
    }

    private static class SingletonHolder {
        private static final HungrySingleton INSTANCE = new HungrySingleton();
    }
}
public class HungrySingleton {
    private HungrySingleton() {

    }

    public static HungrySingleton getInstance() {
        return SingletonHolder.INSTANCE;
    }

    private static class SingletonHolder {
        private static final HungrySingleton INSTANCE = new HungrySingleton();
    }
}

枚举实现 ​

目前最为安全的实现单例的方法是通过内部静态enum的方法来实现，因为JVM会保证enum不能被反射并且构造器方法只执行一次

java
public enum EnumSingleton {
    INSTANCE
}
public enum EnumSingleton {
    INSTANCE
}

单元测试 ​

以DoubleLockSingleton为例

groovy
class DoubleLockSingletonSpec extends Specification {
    def "is DoubleLockSingleton singleton"() {
        given:
        def instance = DoubleLockSingleton.getInstance()
        def anotherInstance = DoubleLockSingleton.getInstance()

        expect:
        instance != null
        anotherInstance != null
        instance == anotherInstance
        instance.is(anotherInstance)
    }
}
class DoubleLockSingletonSpec extends Specification {
    def "is DoubleLockSingleton singleton"() {
        given:
        def instance = DoubleLockSingleton.getInstance()
        def anotherInstance = DoubleLockSingleton.getInstance()

        expect:
        instance != null
        anotherInstance != null
        instance == anotherInstance
        instance.is(anotherInstance)
    }
}