1. Use concurrent collections and atomic variables to reduce competition
In the scenario of concurrent execution of multiple threads, if multiple threads access the same shared resource at the same time, a race condition will occur, which will lead to data inconsistency or program exceptions. Therefore, it is very necessary to use concurrent collections and atomic variables to reduce competition.
1.1 Concurrent collections
Concurrent collection is a thread-safe collection framework provided by Java, including ConcurrentHashMap, ConcurrentLinkedQueue, CopyOnWriteArrayList, etc. They use lock segmentation technology in their implementation. Different elements are assigned to different segments, and different segments can be accessed by different threads at the same time, thereby achieving efficient concurrent operations.
Take ConcurrentHashMap as an example, it is a thread-safe hash table that supports efficient concurrent access. You can use the putIfAbsent() method to add elements to the ConcurrentHashMap. This method checks whether the corresponding key-value pair already exists in the hash table, and adds it if it does not exist. The specific implementation of this method is as follows:
public V putIfAbsent(K key, V value) {
V v = map.get(key);
if (v == null) {
v = map.putIfAbsent(key, value);
}
return v;
}1.2 Atomic variables
Atomic variables are a thread-safe variable type provided by Java, which can ensure that operations on variables are atomic. Common atomic variables include AtomicInteger, AtomicLong, AtomicBoolean, etc.
Taking AtomicInteger as an example, it can be used to replace variables of type int to ensure thread safety in a multi-threaded environment. Atomic operations can be performed using the compareAndSet() method. This method will compare whether the current value is equal to the expected value, and update it to the new value if they are equal. The specific implementation of this method is as follows:
public final boolean compareAndSet(int expect, int update) {
return unsafe.compareAndSwapInt(this, valueOffset, expect, update);
}2. Use thread pool and Future to improve concurrency performance
In high concurrency scenarios, it is necessary to use thread pool and Future to improve concurrency performance.
2.1 Thread pool
The thread pool is an important concurrent programming tool, which can reuse the created threads and avoid the overhead of frequently creating and destroying threads. In Java, you can use the ThreadPoolExecutor class to create a thread pool.
Here is a simple thread pool example:
ExecutorService executorService = Executors.newFixedThreadPool(10);
for (int i = 0; i < 100; i++) {
final int taskIndex = i;
executorService.execute(() -> {
// 执行任务
System.out.println("Task " + taskIndex + " executed.");
});
}
executorService.shutdown();2.2 Future
Future is an asynchronous programming tool provided by Java, which can return a Future object when performing time-consuming operations, through which the operation result can be obtained. In high-concurrency scenarios, using Future can improve the readability and maintainability of the code.
Here is a simple Future example:
ExecutorService executorService = Executors.newFixedThreadPool(10);
Future<Integer> future = executorService.submit(() -> {
// 执行耗时操作
return 1 + 2;
});
int result = future.get(); // 获取操作结果
System.out.println("Result: " + result);
executorService.shutdown();3. Avoid using blocking calls and unnecessary waiting
In high-concurrency scenarios, using blocking calls and unnecessary waiting can lead to program performance degradation. Therefore, it is necessary to avoid using blocking calls and unnecessary waiting as much as possible.
3.1 Avoid blocking calls
In Java, common blocking calls include IO operations, synchronized keywords, Object.wait(), etc. NIO, Lock, Condition, etc. can be used instead of these blocking calls to improve program performance.
3.2 Avoid unnecessary waiting
In the scenario of concurrent execution of multiple threads, unnecessary waiting will lead to a decrease in program performance. You can use Lock and Condition to implement waiting and notification between threads.
Here is a simple Lock and Condition example:
Lock lock = new ReentrantLock();
Condition condition = lock.newCondition();
int count = 0;
Thread t1 = new Thread(() -> {
lock.lock();
try {
while (count == 0) {
condition.await(); // 等待
}
System.out.println("Count: " + count);
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
lock.unlock();
}
});
Thread t2 = new Thread(() -> {
lock.lock();
try {
count = 1;
condition.signal(); // 通知
} finally {
lock.unlock();
}
});
t1.start();
t2.start();Summarize:
This article introduces some commonly used concurrent programming techniques, including using concurrent collections and atomic variables to reduce competition, using thread pools and Futures to improve concurrent performance, avoiding blocking calls and unnecessary waiting, etc. In practical applications, appropriate concurrent programming tools and techniques can be selected according to specific needs, so as to improve program performance and maintainability.