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This article mainly talks about "what are the JUC tools". Interested friends may wish to have a look. The method introduced in this paper is simple, fast and practical. Let's let the editor take you to learn what JUC tools are.

JUC is the abbreviation of java.util.concurrent, and its implementation refers to EDU.oswego.cs.dl.util.concurrent, which is an implementation of the JSR 166standard specification; JSR 166is a specification proposal for Java concurrent programming, which is implemented by the java.util.concurrent package in JDK. That is, JUC is a concurrent package provided by Java, which contains some basic components used in concurrent programming. The classes under the JUC package basically contain some of the tools we use in concurrent programming. It can be roughly divided into the following categories:

Atomic update Java provides java.util.concurrent.atomic packages from JDK1.5, making it easy for programmers to perform atomic operations without locks in a multi-threaded ring. There are 12 classes in the Atomic package and there are four atomic update methods, namely, atomic update basic type, atomic update array, atomic update reference and atomic update field.

The lock and condition variable java.util.concurrent.locks package contains the framework AbstractQueuedSynchronizer of the synchronizer, the Lock based on AQS, and the Condition that works with Lock to implement the wait / notification mode. Most utility classes under JUC use Lock and Condition to achieve concurrency.

The classes involved in the thread pool are Executor, Executors, ThreadPoolExector, AbstractExecutorService, Future, Callable, ScheduledThreadPoolExecutor, and so on.

The classes involved in blocking queues are ArrayBlockingQueue, LinkedBlockingQueue, PriorityBlockingQueue, LinkedBlockingDeque, and so on.

Concurrency containers involve classes such as ConcurrentHashMap, CopyOnWriteArrayList, ConcurrentLinkedQueue, CopyOnWriteArraySet, and so on.

The rest of the synchronizer is a few utility classes that are often used in concurrent programming, mainly to assist thread synchronization. For example: CountDownLatch, CyclicBarrier, Exchanger, Semaphore, FutureTask and so on. Before learning JUC, we need to understand CAS,AQS and Unsafe. For most students who are beginners of concurrent programming, these concepts are easy to be confused, so let's talk about these concepts separately.

Unsafe

Unsafe is a class under the sun.misc package, and it is also an unsafe class. Why would you say that? Java is a very secure language, which can not directly access the bottom of the operating system, but through local methods, but it opens the "back door" to the Unsafe class. The Unsafe class provides hardware-level atomic operations, directly accessing the operating system bottom layer and operating memory. If developers do not use it correctly, it is very easy to cause program errors, program crashes and other problems. Be careful when using Unsafe on a daily basis Don't use it if you don't understand the principle behind it. In Java 9, the official proposal to remove Sun.misc.Unsafe is too wide for the Java language ecology to remove easily. A new jdk.internal.misc.Unsafe class has been added to JAVA 9 to replace the former. The latter is not directly exposed to the application.

This picture is from Meituan's technical team: https://tech.meituan.com/2019/02/14/talk-about-java-magic-class-unsafe.html

CAS

Java provides non-blocking volatile keywords to solve the visibility problem of shared variables, which to some extent makes up for the overhead problem caused by locks, but volatile can only ensure the visibility of shared variables, but can not solve the atomicity problems of read-change-write and so on. CAS (Compare And Swap) refers to comparison and exchange, which is a kind of atomic operation, which can be used to realize uninterrupted data exchange operation in multi-thread programming, so as to avoid the data inconsistency caused by multi-thread rewriting a certain data at the same time due to the uncertainty of execution sequence and the unpredictability of interruption. By comparing the value in memory with the specified data, this operation replaces the data in memory with the new value when the value is the same. The Unsafe class in JDK provides a series of compareAndSwap* methods. The CAS algorithm CAS (V, E, N) contains three parameters, V represents the variable to be updated, E represents the expected value, and N represents the new value. The V value is set to N only if the V value is equal to the E value, and if the V value and E value are different, it means that another thread has been updated and the current thread does nothing. Finally, CAS returns the true value of the current V. All the classes under the Concurrent package are implemented by CAS operations, and sun.misc.Unsafe provides us with a series of CAS operations. There are two common problems with CAS?

ABA problem

Spin problem

AQS

The abstract synchronization queue of AbstractQueuedSynchronizer is referred to as AQS, which is the basic component of the synchronizer. The underlying layer of the lock in the concurrent package is implemented using AQS. AQS defines a synchronization framework for multi-thread access to shared resources, and the implementation of many synchronization classes depends on it, such as Synchronized, ReentrantLock, ReentrantReadWriteLock, Semaphore, CountDownLatch and so on. Locks under this framework will first try to acquire locks with CAS optimistic locks, and if not, they will become pessimistic locks (such as RetreenLock).

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