definition :
thread ： An execution process in a process
The difference between thread and process ： Each process needs an independent memory address space allocated by the operating system , All threads in the same process work in the same address space , These threads can share the same block of memory and system resources .
deadlock ： When a thread waits for a lock held by another thread , While the latter is waiting for a lock already held by the first thread , A deadlock will occur .
Implementation mode :
Java There are two ways to implement multithreading , One is inheritance Thread class , Second, implementation Runnable Interface .
Thread state and its transition :
*
When a thread executes start After method , Does not mean that this thread will be executed immediately , It only means that this thread is in a running state , Finally by OS To determine which runnable thread is executed .
*
There is a time limit for a thread to be selected for execution at one time , This time period is called CPU Time slice of , When the timeslice runs out but the thread is not finished , This thread will become runnable again , wait for OS Rescheduling of ; Execute in running thread Thread.yeild() Method can also make the current thread runnable .
* Waiting for user input in a running thread , call Thread.sleep(), Calling the join() method , The current thread becomes blocked .
* Blocked thread user input completed ,sleep Time out ,join End of thread for , The current thread changes from blocked to runnable .
* Running thread call wait method , This thread enters the waiting queue .
*
Running thread encountered synchronized At the same time, I didn't get the lock mark of the object , Thread waiting for queue wait Time out , The thread waiting for the queue is notify Method wake up , There are other thread calls notifyAll method , Then the thread becomes the lock pool state .
* Thread in lock pool state gets object lock token , The thread becomes runnable .
* Running threads run Method completed or main End of thread , End of thread running .
Method introduction :
* Thread.yield() The currently running thread becomes runnable .
* t2.join() Causes the current thread to block until t2 Thread execution completed .
* Thread.sleep() Causes the current thread to block until sleep End of .
*
wait,notify,notifyAll The way is Object Method of class , Its calling environment must have synchronized Call in synchronization block , Otherwise, it will be thrown java.lang.IllegalMonitorStateException abnormal .
sleep() and yeild() The difference between :
1,sleep() Method gives other threads the chance to run , Regardless of the priority of other threads , So it gives lower threads a chance to run ;yield() Method only gives threads with the same or higher priority a chance to run .
2, When the thread executes sleep(long
millis) After method , Will go to blocked state , parameter millis Specify sleep time ; When the thread executes yield() After method , Will go to ready .
3,sleep() Method declaration throw InterruptedException abnormal , and yield() Method does not declare any exception to be thrown
4,sleep() Method ratio yield() The method has better portability
About object locks :
When a thread attempts to access the synchronized(this) When a code block is marked , Must obtain this Lock of the object referenced by the key , There are two situations as follows :
1,
If the lock is occupied by another thread ,JVM This thread will be put into the lock pool of this object . This thread is blocked . There may be many threads in the lock pool , Wait until the other threads release the lock ,JVM A thread will be randomly removed from the lock pool , Lock this thread , And go to ready .
2, If the lock is not occupied by other threads , This thread will get this lock , Start execution of synchronization code block .
（ In general, the synchronization lock will not be released when the synchronization code block is executed , But there are also special cases where an object lock can be released
Such as when executing synchronization code block , Thread termination due to exception , The lock will be released ; When executing a code block , Of the object to which the lock belongs wait() method , This thread will release the object lock , Enter the waiting pool of the object ）
Characteristics of thread synchronization ：
1, If a synchronized code block and an unsynchronized code block operate on shared resources at the same time , There will still be competition for shared resources . Because when a thread executes an object's synchronization code block , Other threads can still execute unsynchronized blocks of code for objects .（ So called synchronization between threads , When different threads execute the synchronized code block of the same object , To get the synchronization lock of an object and hold each other ）
2, Each object has a unique synchronization lock
3, You can use before static methods synchronized Modifier .
4,
When a thread starts executing a synchronized block of code , Doesn't mean you have to run it all the time , The thread entering the synchronization code block can execute Thread.sleep() Or execution Thread.yield() method , It does not release the object lock at this time , Just give the chance to run to other threads .
5,
Synchronized Declaration will not be inherited , If one uses synchronized Decorated method is covered by subclass , This method is not synchronized in the subclass , Unless synchronized modification .
Thread safe classes ：
1, Objects of this class can be accessed safely by multiple threads at the same time .
2, Each thread can perform atomic operations normally , Get the right results .
3, After the atomic operation of each thread is completed , The object is in a logical and reasonable state .
Release lock on object ：
1, When the synchronization code block is executed, the lock of the object will be released
2, In the process of executing synchronization code block , Thread termination due to exception , The lock will also be released
3, In the process of executing synchronization code block , Of the object to which the lock belongs wait() method , This thread will release the object lock , Enter object's waiting pool .
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