单例模式

理解：在整个项目中，该类的实例只能有一个

1.饿汉式

• 优点：线程安全

• 缺点：浪费资源

public class A {
​private static A a = new A();private A(){}public static A getInstance(){return a;}public static void method(){System.out.println("好好学习，天天向上");}
​public static void main(String[] args) {//      A a1 = A.getInstance();
//      A a2 = A.getInstance();
//      
//      System.out.println(a1 == a2);//trueA.method();//哪怕调用方法也会创建对象开辟空间}

2.饱汉式

• 优点：节约资源

• 缺点：线程不安全

public class A {
​private static A a;private A(){}public static A getInstance(){if(a == null){a = new A();}return a;}public static void method(){System.out.println("好好学习，天天向上");}
}
​
​
public static void main(String[] args) {A a1 = A.getInstance();A a2 = A.getInstance();System.out.println(a1 == a2);//true//      A.method();}

3.双重校验

• 优点：节约资源、线程安全

public class A {//创建一个对象的步骤：A a = new A();//1.创建对象空间，分配地址 -- new --> 0x001//2.调用构造方法，初始化成员变量//3.将对象地址赋值给引用//注意：创建对象的步骤有可能是1、2、3，也有可能是1、3、2//注意：使用volatile修饰的对象被创建的步骤必须是1、2、3
​private static volatile A a;private A(){}public static A getInstance(){if(a == null){synchronized (A.class) {if(a == null){a = new A();}}}return a;}public static void method(){System.out.println("好好学习，天天向上");}
​public static void main(String[] args) {A a1 = A.getInstance();A a2 = A.getInstance();System.out.println(a1 == a2);//true//      A.method();}    

死锁

• 注意：死锁不一定每次都出现

• 经验：尽可能避免锁嵌套

public class Test01 {
​public static void main(String[] args) {new Thread(new Runnable() {@Overridepublic void run() {synchronized (KuaiZi.a) {try {Thread.sleep(1);} catch (InterruptedException e) {e.printStackTrace();}synchronized (KuaiZi.b) {System.out.println("哲学家1发现了");}}}}).start();new Thread(new Runnable() {@Overridepublic void run() {synchronized (KuaiZi.b) {try {Thread.sleep(1);} catch (InterruptedException e) {e.printStackTrace();}synchronized (KuaiZi.a) {System.out.println("哲学家2发现了");}}}}).start();}
}
class KuaiZi{public static Object a = new Object();public static Object b = new Object();}