关于线程安全的一些小问题

 Person 类代码如下

public class Person {
    private int age;
    
    private String name;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }


}

 

public class Easy {

    private Person person = new Person();

    private static volatile Easy easy = null;

    private Easy() {
    }


    public static Easy instance() {
        if (easy == null) {
            synchronized (Easy.class) {
                if (easy == null) {
                    easy = new Easy();
                }
            }
        }
        return easy;
    }


    public void printPerson() {
        System.out.println(person);
    }


    public static void main(String[] args) throws Exception {
        new Thread(new Runnable() {
            @Override
            public void run() {
                Easy.instance().printPerson();
            }
        }).start();
        new Thread(new Runnable() {
            @Override
            public void run() {
                Easy.instance().printPerson();
            }
        }).start();
    }
}

运行结果：

com.example.task.Person@6ab31cd3
com.example.task.Person@6ab31cd3

典型的火车买票的代码，这里这个person是线程共享的，存在堆内存中。

 

 

public class Easy {

    private Person person = null;

    private static volatile Easy easy = null;

    private Easy() {
    }

    public static Easy instance() {
        if (easy == null) {
            synchronized (Easy.class) {
                if (easy == null) {
                    easy = new Easy();
                }
            }
        }
        return easy;
    }


    public void printPerson() {
        System.out.println(person);
        person = new Person();
        System.out.println(person);
    }


    public static void main(String[] args) throws Exception {
        new Thread(new Runnable() {
            @Override
            public void run() {
               Easy.instance().printPerson();
            }
        }).start();
        new Thread(new Runnable() {
            @Override
            public void run() {
                Easy.instance().printPerson();
            }
        }).start();
    }
}

 运行结果：

null
com.example.task.Person@28979470
com.example.task.Person@28979470
com.example.task.Person@26507f12

 

从这里看到，如果在线程一中对成员变量进行赋值，切换到线程二的时候引用还是线程一的。

 

 

对于下面这段代码，为什么永远是同一个对象呢

 @Autowired
 private DialogueManager dialogueManager;

 这是因为工厂（spring工厂）获取的永远是用一个对象，使用代码可以这样表示

 

 

package com.example.task;

public class Person {
    private static volatile Person person = null;

    private Person() {
    }

    public static Person instance() {
        if (person == null) {
            synchronized (Person.class) {
                if (person == null) {
                    person = new Person();
                }
            }
        }
        return person;
    }

    private String name;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    private int age;
}

 

package com.example.task;


public class Easy {

    private Person person = null;

    private static volatile Easy easy = null;

    private Easy() {
    }


    public static Easy instance() {
        if (easy == null) {
            synchronized (Easy.class) {
                if (easy == null) {
                    easy = new Easy();
                }
            }
        }
        return easy;
    }


    public void printPerson() {
        System.out.println(person);
        person = Person.instance();
        System.out.println(person);
    }


    public static void main(String[] args) throws Exception {
        new Thread(new Runnable() {
            @Override
            public void run() {
               Easy.instance().printPerson();
            }
        }).start();
        new Thread(new Runnable() {
            @Override
            public void run() {
                Easy.instance().printPerson();
            }
        }).start();
    }
}

 运行结果：

 

null
com.example.task.Person@4f114b68
com.example.task.Person@4f114b68
com.example.task.Person@4f114b68