静态变量方式
public class Singleton {
private static Singleton singleton = new Singleton();
private Singleton() {}
public static Singleton getInstance() {
return singleton;
}
}
1.2 饿汉式-2
静态代码块方式
public class Singleton {
private static Singleton singleton;
private Singleton() {}
static {
singleton = new Singleton();
}
public static Singleton getInstance() {
return singleton;
}
}
1.3 饿汉式-3
枚举方式
public enum Singleton {
SINGLETON
}
1.4 懒汉式-1
线程不安全
public class Singleton {
private static Singleton singleton = null;
private Singleton() {}
public static Singleton getInstance() {
if (singleton == null) {
singleton = new Singleton();
}
return singleton;
}
}
1.5 懒汉式-2
线程安全-1
public class Singleton {
private static Singleton singleton = null;
private Singleton() {}
public static synchronized Singleton getInstance() {
if (singleton == null) {
singleton = new Singleton();
}
return singleton;
}
}
1.6 懒汉式-3
线程安全-2
public class Singleton {
private static volatile Singleton singleton;
private Singleton() {}
public static Singleton getInstance() {
if (singleton == null) {
synchronized (Singleton.class) {
if (singleton == null) {
singleton = new Singleton();
}
}
}
return singleton;
}
}
1.7 懒汉式-4
静态内部类方式
public class Singleton {
private Singleton() {}
private static class SingletonHolder {
private static final Singleton SINGLETON = new Singleton();
}
public static Singleton getInstance() {
return SingletonHolder.SINGLETON;
}
}
2、破坏单例模式 2.1 序列化方式
以静态内部类为例
public class Singleton implements Serializable {
private Singleton() {}
private static class SingletonHolder {
private final static Singleton SINGLETON = new Singleton();
}
public static Singleton getInstance() {
return SingletonHolder.SINGLETON;
}
}
测试类：
public class Client {
public static void main(String[] args) throws Exception {
writeObject();
readObject();
readObject();
}
public static void readObject() throws Exception {
ObjectInputStream objectInputStream =
new ObjectInputStream(new FileInputStream("D:/桌面/1.txt"));
Singleton singleton = (Singleton) objectInputStream.readObject();
System.out.println(singleton);
objectInputStream.close();
}
public static void writeObject() throws Exception {
Singleton instance = Singleton.getInstance();
ObjectOutputStream objectOutputStream =
new ObjectOutputStream(new FileOutputStream("D:/桌面/1.txt"));
objectOutputStream.writeObject(instance);
objectOutputStream.close();
}
}
运行结果：
2.2 反射方式
以静态内部类为例
public class Singleton implements Serializable {
private Singleton() {}
private static class SingletonHolder {
private final static Singleton SINGLETON = new Singleton();
}
public static Singleton getInstance() {
return SingletonHolder.SINGLETON;
}
}
测试类：
public class Client2 {
public static void main(String[] args) throws Exception {
// 获取 class 对象
Class singletonClass = Singleton.class;
// 获取无参构造方法
Constructor constructor = singletonClass.getDeclaredConstructor();
// 取消访问检查
constructor.setAccessible(true);
// 创建两个对象
Singleton singleton1 = constructor.newInstance();
Singleton singleton2 = constructor.newInstance();
System.out.println(singleton1 == singleton2);
}
}
运行结果：
3、解决单例模式被破坏 3.1 序列化方式
添加 readResolve() 方法
public class Singleton implements Serializable {
private Singleton() {}
private static class SingletonHolder {
private final static Singleton SINGLETON = new Singleton();
}
public static Singleton getInstance() {
return SingletonHolder.SINGLETON;
}
public Object readResolve() {
return SingletonHolder.SINGLETON;
}
}
3.2 反射方式
通过标志位
public class Singleton implements Serializable {
private static boolean flag = false;
private Singleton() {
synchronized (Singleton.class) {
if (flag == true) {
throw new RuntimeException("不要试图利用反射破坏单例");
}
flag = true;
}
}
private static class SingletonHolder {
private final static Singleton SINGLETON = new Singleton();
}
public static Singleton getInstance() {
return SingletonHolder.SINGLETON;
}
}
