Java locks (Java Locks) are mechanisms used in the Java programming language to achieve multi-thread synchronization and mutual exclusion. In concurrent programming, simultaneous access to shared resources by multiple threads may lead to race conditions (Race Condition) and other concurrency issues. Java locks provide a way to control concurrent access by multiple threads to ensure thread safety (Thread Safety) and correctness. data access.
Java locks play an important role in Java multithreaded programming. Java provides various types of locks, such as the synchronized keyword, ReentrantLock class, Read/Write Locks, etc., to meet the concurrency control requirements in different scenarios. The correct use of Java locks can avoid race conditions, deadlocks and other concurrency problems among multi-threads, and ensure the correctness and stability of multi-threaded programs.
This article will deeply discuss the principle, usage, performance characteristics, common problems and solutions of Java locks, etc., to help readers understand the concept and application of Java locks.
1. Overview of Java locks
Java lock is a multi-thread synchronization mechanism used to control the concurrent access of multiple threads to shared resources. The role of Java locks is to ensure mutual exclusion between threads (Mutual Exclusion), that is, only one thread can access shared resources at the same time, thereby avoiding race conditions (Race Condition) and other concurrency problems between multiple threads.
Java locks can be divided into two categories: implicit locks (Implicit Locks) and explicit locks (Explicit Locks).
Implicit locks , also known as built-in locks (Intrinsic Locks) or synchronized locks, are a lock mechanism provided at the Java language level. By using the synchronized keyword in a method or code block, the Java compiler and JVM will automatically add a lock on the object or class to achieve synchronized access to shared resources. The use of implicit locks is simple and convenient, but the granularity of locks is relatively coarse, and only basic mutual exclusion and synchronization can be achieved.
Explicit locks , also known as external locks (Explicit Locks), are implemented through the Lock interface and its implementation classes in the Java language. Explicit locks provide more flexible and fine-grained lock control, such as reentrancy, condition variables, fairness, and more. The use of explicit locks requires explicit acquisition and release of locks, which provides more operation and status information, and is suitable for complex concurrency control scenarios.
Java locks play an important role in multi-threaded programming. They can achieve thread-safe access to shared resources, protect the integrity and correctness of shared resources, and avoid race conditions and other concurrency issues between multiple threads.
Two, Java implicit lock (synchronized)
Java implicit lock, also known as built-in lock or synchronized lock, is a simple and convenient lock mechanism provided by the Java language, which can realize synchronized access to shared resources by using the synchronized keyword in methods or code blocks.
2.1 synchronized keyword
The synchronized keyword can modify methods, instance objects or class objects, and is used to synchronize access to shared resources in a multi-threaded environment.
a. Modified method: add the synchronized keyword before the method signature, indicating that the entire method body is a synchronized code block, and the object's lock will be automatically acquired when the method is called.
public synchronized void synchronizedMethod() {
// 同步代码块
}
b. Modify the instance object: Use the synchronized keyword to modify the code block, specify the locked object, and only the thread that obtains the lock of the object can execute the code block.
public void someMethod() {
synchronized (this) {
// 同步代码块
}
}
c. Modify the class object: Use the synchronized keyword to modify the static method, indicating that the entire static method body is a synchronized code block, and the lock of the class object will be automatically acquired when the static method is called.
public static synchronized void synchronizedStaticMethod() {
// 同步代码块
}
2.2 Characteristics of implicit locks
The characteristics of implicit locks are as follows:
a. Mutual Exclusion: Only one thread can hold the lock at the same time, and other threads cannot obtain the lock, thus ensuring mutually exclusive access to shared resources.
b. Reentrant: The same thread can acquire the lock multiple times without causing deadlock.
c. Non-Fairness: The implicit lock is an unfair lock by default, that is, the order in which threads acquire locks is not guaranteed to be the same as the order in which they request locks, which may cause some threads to be unable to acquire locks for a long time.
d. Release Condition (Release Condition): The implicit lock is automatically released. When the thread exits the synchronization code block, the lock will be released automatically. It can also be explicitly called by calling wait(), notify(), notifyAll(), etc. release the lock.
2.3 Precautions for using implicit locks
When using implicit locks, you need to pay attention to the following points:
a. Object-level lock: The method or code block modified by the synchronized keyword is an object-level lock by default, that is, each object instance has its own lock, and different object instances do not affect each other.
b. Class-level lock: The static method or code block modified by the synchronized keyword is a class-level lock, that is, all object instances share the same lock.
c. Lock granularity: The granularity of implicit locks is an important factor to consider. If the granularity of the lock is too large, that is, the entire object or the entire method is locked, it may cause a performance bottleneck, because multiple threads cannot be executed concurrently, thereby reducing the throughput of the system. And if the granularity of the lock is too small, that is, too many small code blocks are locked, it may cause frequent lock competition and reduce system performance. Therefore, when using implicit locks, it is necessary to choose a reasonable lock granularity to balance the relationship between concurrency and performance.
d. Nesting of locks: When using implicit locks, you need to pay attention to the nesting of locks, that is, whether a lock can be acquired again within a lock. Locks in Java are reentrant, and the same thread can acquire the same lock multiple times without deadlock. However, it should be noted that the use of nested locks should be cautious to avoid deadlocks or other concurrency problems.
e. Lock release: The implicit lock is automatically released, that is, the lock is automatically released when the synchronization code block is executed or exits abnormally. However, if methods such as wait(), notify(), notifyAll() are used inside the synchronized code block, the lock needs to be released explicitly, otherwise it may cause deadlock or other concurrency problems.
2.4 Advantages and disadvantages of implicit locks
As the most basic locking mechanism in Java, implicit lock has the following advantages:
a. Easy to use: The synchronized keyword is a built-in lock provided by the Java language. It is simple and convenient to use, and does not need to explicitly create a lock object or call lock-related methods.
b. Easy to debug: Implicit lock is a native lock provided by the Java language, which can easily add debugging information or logs to the code to facilitate troubleshooting of concurrency problems.
c. Support reentrancy: implicit locks support reentrancy of threads to the same lock without causing deadlock.
d. Support automatic release: The implicit lock will automatically release the lock when the synchronization code block is executed or exits abnormally, and there is no need to release it manually.
However, implicit locks also have some disadvantages:
a. Unfairness: Implicit locks are unfair locks by default, which may cause some threads to be unable to acquire locks for a long time, thus affecting system performance.
b. Large granularity: The granularity of implicit locks is large, which may result in the inability to execute concurrently among multiple threads, thereby reducing the throughput of the system.
c. Lock restrictions: implicit locks can only modify methods, instance objects or class objects, and cannot synchronize other objects.
2.5 Display Lock
Explicit locks are implemented through the Lock interface in Java and its implementation classes, which provide a more flexible and powerful locking mechanism, and have more advantages than implicit locks.
a. Fairness: Unlike implicit locks, explicit locks can support fairness, that is, locks are acquired in the order in which threads are requested to avoid the problem that some threads cannot acquire locks for a long time.
b. Controllable granularity: Explicit locks can manually control the acquisition and release of locks through the lock() and unlock() methods, so that the granularity of locks can be controlled more finely, and the problem of too large or too small granularity can be avoided.
c. Interruptible: Explicit locks provide a mechanism that can interrupt waiting for locks. Through the lockInterruptibly() method, interrupts can be responded to while waiting for locks, thereby avoiding long-term blocking of threads.
d. Support multiple conditions: Explicit locks can support multiple conditional waiting and wake-up through the Condition object, so that more complex thread cooperation mechanisms can be realized.
e. High performance: Explicit locks can have better performance than implicit locks in some cases, because it provides more optimization options, such as reentrant locks, read-write locks, etc.
2.6 Example of display lock usage
Here is an example using explicit locks:
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
public class LockExample {
private Lock lock = new ReentrantLock(); // 创建显式锁
public void doSomething() {
lock.lock(); // 获取锁
try {
// 执行需要同步的代码
} finally {
lock.unlock(); // 释放锁
}
}
}
In the above example, the implementation class ReentrantLock of the Lock interface is used to create an explicit lock. Acquire the lock by calling the lock() method, execute the code that needs to be synchronized, and finally release the lock by calling the unlock() method in the finally block. This method can manually control the acquisition and release of locks, so as to achieve finer-grained concurrency control.
Summarize
The lock in Java is an important tool for concurrency control, which can help developers solve the problem of multi-threaded concurrent access to shared resources. Implicit locks provide a simple and easy-to-use locking mechanism through the synchronized keyword, but there may be disadvantages such as unfairness and large granularity. Explicit locks provide a more flexible and powerful locking mechanism through the Lock interface and its implementation classes, which can support features such as fairness, controllable granularity, and interruptibility, but manual control of lock acquisition and release is required. In actual projects, choosing an appropriate locking mechanism should be based on specific needs and scenarios, taking into account factors such as fairness, performance, and granularity.
At the same time, when using locks, you need to follow some best practices to ensure thread safety and efficient concurrency control, as follows:
- Avoid excessive lock contention: Lock contention is one of the main causes of performance degradation. Therefore, when designing concurrent code, you should minimize the use of locks, and try to use lock-free or low-lock methods to achieve concurrency control.
- Use the smallest granularity of locks: The smaller the granularity of the lock, the more threads that can be executed at the same time, thereby improving concurrency performance. Therefore, you should try to use the smallest granularity of locks, avoid locking the entire method or class, but only lock the necessary shared resources.
- Avoid deadlocks: Deadlocks are a common concurrency programming error that cause threads to wait for each other without further execution. Therefore, when writing concurrent code, be careful to avoid deadlock situations, such as avoiding requesting the same lock again while holding a lock, and reasonably designing the order of lock acquisition and release.
- Pay attention to the lock release: The lock release should be placed in a suitable position to prevent problems caused by premature or late release of the lock. The finally block is usually used to ensure the release of the lock so that the lock can be released correctly when an exception occurs.
- Use an appropriate lock mechanism: Choose an appropriate lock mechanism based on specific needs and scenarios, such as using the synchronized keyword to achieve simple synchronization, or use explicit locks to achieve more complex concurrency control. When choosing an explicit lock, factors such as fairness, granularity, and interruptibility should also be considered.
- Consider performance and scalability: Concurrency control should not only consider thread safety, but also consider performance and scalability. Therefore, when designing concurrent code, it is necessary to comprehensively consider performance and scalability factors to avoid performance bottlenecks or poor scalability.
- Conduct multi-thread testing: The correctness of concurrent code is often difficult to verify. Therefore, after writing concurrent code, full multi-thread testing should be performed to simulate different concurrency scenarios and loads to ensure the correctness and stability of concurrent code.
- Do not abuse locks: Locks are a powerful tool, but they are also a mechanism that consumes a lot of resources. Abuse of locks may lead to performance degradation and poor concurrency. Therefore, you should avoid using locks in unnecessary places, use them only when necessary, and reasonably evaluate the impact of locks on performance and concurrency performance.
- Use concurrent containers: Java provides some concurrent containers, such as ConcurrentHashMap, ConcurrentLinkedQueue, etc., which provide efficient concurrent access in a multi-threaded environment. Using these concurrent containers can avoid implementing the locking mechanism by yourself, thus simplifying the complexity of concurrent programming.
- Consider the reentrancy of locks: Locks in Java are reentrant, that is, the same thread can acquire the same lock multiple times without deadlock. When writing concurrent code, pay attention to the reentrancy of locks to avoid deadlocks.
- Use appropriate inter-thread communication methods: In concurrent programming, inter-thread communication is an important issue. Java provides a variety of ways to communicate between threads, such as the synchronized keyword, wait(), notify(), notifyAll(), etc. When using inter-thread communication, choose the appropriate method to ensure that threads can work together correctly.
- Consider the granularity and level of locks: When designing concurrent code, the granularity and level of locks should be considered reasonably. Coarse-grained locks may lead to poor concurrency performance, while fine-grained locks may lead to excessive locking overhead. Therefore, it is necessary to select the appropriate lock granularity and level according to specific needs and scenarios.
- Avoid using outdated locking mechanisms: Java provides a variety of locking mechanisms, such as the synchronized keyword, ReentrantLock, ReadWriteLock, etc. Some old lock mechanisms such as Vector and Hashtable perform poorly in concurrency performance, so you should try to avoid using these outdated lock mechanisms.
- Consider scalability: As hardware technology advances, modern computer systems increasingly feature multi-core processors and multiple processors. Therefore, when designing concurrent code, the scalability of the system should be considered to fully utilize the parallel performance of multi-core processors and multi-processors.
- Learn the best practices of concurrent programming: Concurrent programming is a complex technique that requires an in-depth understanding of Java's concurrency mechanisms and best practices. Learning the best practices of concurrent programming, understanding and mastering the relevant knowledge and technologies of Java concurrent programming is the key to writing efficient and thread-safe concurrent code. Continuously learning the best practices of concurrent programming, referring to industry experience and cases, and accumulating practical experience in actual projects can help developers better design and implement high-performance, high-concurrency Java applications.