The thread pool

  • Threads are precious memory resources. A single thread occupies 1MB of space. Excessive allocation may cause memory overflow
  • Frequent creation and destruction of threads increases vm reclamation frequency, resource overhead, and program performance degradation
  • Hence the thread pool

The concept of thread pools

  • A thread container that sets an upper limit on the number of threads allocated
  • Pre-created thread objects are pooled and reused from the thread pool
  • Avoid frequent creation and destruction

Principle of thread pool

Getting the thread pool

Creating a thread pool

public class TestThreadPool { public static void main(String[] args) { // 1. Creates a fixed number of threads thread pool objects / / thread pool can exist four threads ExecutorService es = Executors. NewFixedThreadPool (4); Runnable Runnable = new Runnable() {private int ticket = 100; // 2. @Override public void run() { while (true) { if(ticket <= 0) { break; } system.out.println (thread.currentThread ().getName() + "buy" + ticket--); }}}; For (int I = 0; i < 5; i++) { es.submit(runnable); } // 4. Close the thread pool es.shutdown(); }}Copy the code

The execution result

note

/ / 1. Create a single thread thread pool ExecutorService es = Executors. NewSingleThreadExecutor (); / / 1. Create a buffer pool, the number of threads by the task to decide the ExecutorService es = Executors. NewCachedThreadPool ();Copy the code

Callable interface

public interface Callable<V>{
	public V call() throws Exception;
}

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  • JDK5 is added, similar to the Runnable interface, to represent a thread task when implemented
  • Callable has generic return values and can declare exceptions

The Future interface

  • Concept: Asynchronously accept the status result returned by executorService.submit (), which includes the return value from Call ()
  • Method :V get() blocks for the result of asynchronous processing in the Future (return value of call())

The sample

  • Use Future and Callable interfaces
  • Using two threads, calculate the sum of 1-50 and 51-100 simultaneously, and then summarize
Public class TestCallable {public static void main(String[] args) throws Exception {// 1. Create a thread pool objects ExecutorService es = Executors. NewFixedThreadPool (2); // 2. Submit the task and get the Future object. Future<Integer> FUture1 = es. Submit (new Callable<Integer>() {@override public Integer Call () throws System.out.println("start 1-50 count... ); int sum = 0; for(int i = 1; i <= 50; i++) { sum += i; } return sum; }}); Future<Integer> future2 = es.submit(new Callable<Integer>() { @Override public Integer call() throws Exception { System.out.println("start 51-100 count...") ); int sum = 0; for(int i = 51; i <= 100; i++) { sum += i; } return sum; }}); System.out.println(" + (future1.get() + future2.get())) "); // 4. Close the resource es.shutdown(); }}Copy the code

The results of

The Lock interface

  • JDK5 added, compared with synchronized, display definition, structure more flexible
  • Provide more practical methods, more powerful functions, better performance

Commonly used method

Void lock() // Try to acquire the lock(successful true, failed false, not blocked) Boolean tryLock // Release the lock void unlock()Copy the code
// Example public class TestLock implements Runnable {Lock l = new ReentrantLock(); private int ticket = 100; @Override public void run() { while (true) { l.lock(); try { if(ticket <= 0) { break; } System.out.println(Thread.currentThread().getName() + " sells " + ticket--); } catch (Exception e) { e.printStackTrace(); }finally { l.unlock(); } } } public static void main(String[] args) { ExecutorService es = Executors.newFixedThreadPool(4); for(int i = 0; i < 4; i++) { es.submit(new TestLock()); } es.shutdown(); }}Copy the code

The execution result

Thread-safe collection

Tool methods in Collections

Queue interface (Queue) 

Public class TestQueue {public static void main(String[] args) {// 1. PriorityQueue<String> q = new PriorityQueue<String>(); Q.ffer ("a"); q.offer("b"); q.offer("c"); q.offer("d"); q.offer("e"); System.out.println(q.prinl ()); // 3. System.out.println(q.poll()); System.out.println(q.poll()); System.out.println(q.poll()); System.out.println(q.poll()); }}Copy the code

The execution result

The last

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