What is a memory model and why is it needed?

JAVA Memory Model

In this series of multi-threading, we will not explore the bottom layer of the memory model

1. What is the memory model and why is it needed

In modern multi-core processors, each processor has its own cache, which is regularly coordinated with the main memory;

If you want to ensure that each processor knows what other processors are doing at any time, you will Requires a lot of overhead; and is usually unnecessary

We only need to know information when we need to share data across threads; in JAVA this is achieved through correct synchronization

1. Reordering

As follows: It will be very difficult to judge the output value

public class PossibleReordering {static int x = 0, y = 0;static int a = 0, b = 0;/** * 判断输出值将会非常困难：
     * 1：多线程之间的切换，导致可能的输出值：(0,1)(1,0)(1,1)
     * 2.指令重排序：one线程如a=1和x=b之间重排序，x=b(0)，然后other线程被调度执行y=a(0),将导致(0，0)     */public static void main(String[] args) throws InterruptedException {
        Thread one = new Thread(new Runnable() {public void run() {
                a = 1;
                x = b;
            }
        });
        Thread other = new Thread(new Runnable() {public void run() {
                b = 1;
                y = a;
            }
        });
        one.start();
        other.start();
        one.join();
        other.join();
        System.out.println("( " + x + "," + y + ")");
    }
}

2. Introduction to JAVA memory model

Partial ordering relationship: anti-symmetric, reflexive and transitive properties; but for any two elements A and B, it does not necessarily satisfy the relationship that A favors B or B favors A

　　For example: between A and B I prefer B, but I don’t need to make an explicit choice

JMM defines a partial ordering relationship for all operations in the program, called Happens-Before; if you want to ensure that the thread executing B operation, see To the result of the thread that performs operation A, regardless of whether AB is in the same thread, it must satisfy the Happens-Before relationship, otherwise the JVM will reorder it

　　

For example: locking operation can predict the execution order, and multiple threads comply with Happens-Before. Without locking, it is impossible to judge the scheduling between threads,

3. Release

The real reason: There is no Happens-Before relationship between publishing a shared object and accessing the object in another thread; due to the reordering of instructions, the object is released without being constructed correctly

public class UnsafeLazyInitialization {private static Resource resource;/** * 除了竟态条件问题检查后执行，还有不安全发布的问题
     * 如：一个线程A进来，看到resource为null，则实例化并返回；另一个线程B进来看到resource不为null直接返回
     * 如果在线程A中对resource进行了修改，则可能在线程B中看不到resource的正确状态     */public static Resource getInstance() {if (resource == null)
            resource = new Resource(); // unsafe publicationreturn resource;
    }static class Resource {
    }
}

　　

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