图解Java多线程条记：http://tutorials.jenkov.com/java-concurrency/java-memory-model.html

Java内存模子（JMM）界说了：how and when different threads can see
values written to shared variables by other threads,
and how to synchronize access to shared variables when necessary.

Java堆和栈中的工具存储位置：

Java内存模子与硬件模子：

线程读取主内存的数据到CPU缓冲中，劳务派遣管理系统，当数据放在差异位置时，会有两个问题：可见性与静态条件

A synchronized block in Java is synchronized on some object.
All synchronized blocks synchronized on the same object can only
have one thread executing inside them at the same time.
All other threads attempting to enter the synchronized block are blocked
until the thread inside the synchronized block exits the block.
The synchronized keyword can be used to mark four different types of blocks:

1. Instance methods -> on the instance (object) owning the method
2. Static methods -> on the class object of the class belongs to …
3. Code blocks inside instance methods
4. Code blocks inside static methods

Synchronized Instance methods（实例要领的同步）：

静态要领的同步：

代码块的同步：

用jstack查察，同一个监督器工具只答允有一个线程会见：

实例要领的同步加上代码块this的同步，软件开发，仍然针对同一个实例工具：

自界说监督器工具：

同一个实例工具的加锁：

差异实例工具的加锁：

Volatile keyword guarantees visibility of changes to variables across threads.

every read of a volatile variable will be
read from the computer’s main memory,
and not from the CPU cache.
every write to a volatile variable will be
written to main memory,
and not just to the CPU cache.

If Thread A writes to a volatile variable and Thread B subsequently reads the same volatile variable, then all variables visible to Thread A before writing the volatile variable, will also be visible to Thread B after it has read the volatile variable.
The reading and writing instructions of volatile variables cannot be reordered by the JVM. Instructions before and after can be reordered, but the volatile read or write cannot be mixed with these instructions. Whatever instructions follow a read or write of a volatile variable are guaranteed to happen after the read or write.
volatile变量不担保事务：

volatile变量仍然会存在竞态条件：