Java内存模型（JMM）定义了：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堆和栈中的对象存储位置：
Java内存模型与硬件模型：
线程读取主内存的数据到CPU缓冲中，当数据放在不同位置时，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:
- Instance methods -> on the instance (object) owning the method
- Static methods -> on the class object of the class belongs to …
- Code blocks inside instance methods
- Code blocks inside static methods
Synchronized Instance methods（实例方法的同步）：
静态方法的同步：
代码块的同步：
用jstack查看，同一个监视器对象只允许有一个线程访问：
实例方法的同步加上代码块this的同步，仍然针对同一个实例对象：
自定义监视器对象：
同一个实例对象的加锁：
不同实例对象的加锁：
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变量不保证事务：
volatile变量仍然会存在竞态条件：
volatile变量会禁止重排序：
如果变量在volatile变量更新之后，不保证写到主存：
为了保证可见性，不需要为每个变量都定义为volatile类型：
volatile变量是个内存屏障，在这之前和之后的指令可以重排序：
本地线程的示例：
下面的上图没有使用本地线程，下图使用了本地线程：
线程的信号量实现方式–busy waiting：
或者可以用volatile变量：
wait和notify的示例：
notify与notifyAll的示例：
等待线程有可能意外被唤醒，需要用while循环继续判断是否被唤醒线程notify：
一次唤醒所有线程，或者每次一个个地唤醒：
不同线程之间采用字符串作为监视器锁，会唤醒别的线程：
不同线程之间的信号没有共享，等待线程被唤醒后继续进入wait状态：
不同线程的等待与唤醒示例：
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