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This article mainly introduces "how to understand Java concurrent programming". In daily operation, I believe many people have doubts about how to understand Java concurrent programming. Xiaobian consulted all kinds of materials and sorted out simple and easy operation methods. I hope to help you answer the doubts of "how to understand Java concurrent programming"! Next, please follow the small series to learn together!

Locks are used to control how multiple threads access a shared resource. Generally, a lock prevents multiple threads from accessing a shared resource simultaneously (although some locks allow multiple threads to access a shared resource concurrently, such as read-write locks).

happens-before presentation

Starting with JDK 5, Java uses the new JSR-133 memory model (unless otherwise noted, this article focuses on the JSR-133 memory model). JSR-133 uses the concept of happens-before to illustrate memory visibility between operations. In JMM, if the result of an operation is to be visible to another operation, there must be a happens-before relationship between the two operations. The two operations mentioned here can be either within a thread or between different threads

Program ordering rules: Every operation in a thread happens-before any subsequent operation in that thread.

Monitor lock rule: unlocking of a lock happens-before subsequent locking of the lock.

volatile variable rule: Write to a volatile field happens before any subsequent read to the volatile field.

·Transitivity: If A happens-before B and B happens-before C, then A happens-before C.

Note that having a happens-before relationship between two operations does not mean that the first operation must be performed before the second! happens-before only requires that the first action (the result of execution) be visible to the second action, and that the first action be visible to and ordered before the second action.

3.2 reordering

Reordering is a means by which compilers and processors reorder instruction sequences to optimize program performance.

3.2.1 Data dependencies

If two operations access the same variable, and one of the two operations is a write operation, then there is a gap between the two operations

there are data dependencies.

as-if-serial semantics

as-if-serial semantics means: no matter how reordered (compiler and processor to improve parallelism),(single thread)

The execution result of the program cannot be changed

Sequentially consistent memory model

In fact, what the thread sees is a single execution order, i++ is a non-atomic operation compiled into 4 steps, and the execution order is variable.

Data is transferred between the processor and memory via the bus. Each data transfer between processor and memory is accomplished through a series of steps called a bus transaction.

Questions? Is this bus the same concept as the CPU bus? The macro effect is similar

As shown in the picture, it comes from Station B Baiyi

volatile prohibits instruction rearrangement because LoadStore causes it to fail rearrangement

Summarize the memory semantics of fair and unfair locks.

When both fair and unfair locks are released, a volatile variable state is written at the end.

When acquiring a fair lock, the volatile variable is read first.

When acquiring an unfair lock, the volatile variable is first updated with CAS, which has both volatile read and volatile write memory semantics.

thread's priority

Deamon thread

Daemon threads are a supportive thread because they are primarily used for background scheduling and support work in programs. This means that when there are no non-Daemon threads in a Java virtual machine, the Java virtual machine will exit. You can set a thread to a Daemon thread by calling Thread.setDaemon(true).

1. When the main thread exits, will the waiting child thread finish executing?

Not necessarily implemented

ti.setDaemon(true);

Waiting thread execution depends on execution time

understand interrupt

Thread.interrupt() Set state

isInterrupted() returns Boolean

interrupted

Interrupt can be understood as an identification bit attribute of a thread, which indicates whether a running thread has been interrupted by other threads. An interrupt is like another thread saying hello to the thread, which interrupts it by calling its interrupt() method.

The thread responds by checking whether it is interrupted. The thread determines whether it is interrupted by the method isInterrupted(), or it can call the static method Thread.interrupted() to reset the interrupt flag of the current thread.

Thread Pool Technology and Its Examples

ThreadPoolExecutor source code

//Executes a job that requires Runnablepublic void execute(Runnable command) {} //Closes the thread pool public void shutdown() {} Handles the waiting task and returns the task list public List shutdownNow() {}//1) increments the number of threads by 1 using the loop CAS operation;2) creates a new thread and enables it. private boolean addWorker(Runnable firstTask, boolean core) {}

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