多线程并发时，poolSize对执行时间的影响

今天药忘吃喽~ 2022-03-31 04:24 227阅读 0赞

这两天在学习多线程并发，写了一个例子

import java.util.concurrent.Callable;
    import java.util.concurrent.CountDownLatch;
    import java.util.concurrent.ExecutorService;
    import java.util.concurrent.Executors;
    import java.util.concurrent.FutureTask;
    
    public class CountDownLatchTest {
    	private static int poolSize = 2;
    	private static CountDownLatch countDown = new CountDownLatch(poolSize);
    	
    	public static void main(String [] args) {
    
    		long startTime = System.currentTimeMillis();
    		System.out.println("主线程start use:" + startTime);
    		Callable<String> call1 = new Callable<String>() {
    		    @Override  
    		    public String call() throws Exception {  
    		        Thread.sleep(1000);  
    		        countDown.countDown();
    				System.out.println(Thread.currentThread().getName()+" user time "+(System.currentTimeMillis()-startTime));
    		        //return the thread name executing this callable task  
    		        return Thread.currentThread().getName();  
    		    } 
    			
    		};
    		Callable<String> call2 = new Callable<String>() {
    		    @Override  
    		    public String call() throws Exception {  
    		        Thread.sleep(1000); 
    		        countDown.countDown();  
    		        System.out.println(Thread.currentThread().getName()+" user time "+(System.currentTimeMillis()-startTime));
    		        //return the thread name executing this callable task  
    		        return Thread.currentThread().getName();  
    		    } 
    			
    		};
    		
    		FutureTask<String> ft1 = new FutureTask<String>(call1);
    		FutureTask<String> ft2 = new FutureTask<String>(call2);
    		
    		ExecutorService executor = Executors.newFixedThreadPool(poolSize);
    		
    		executor.execute(ft1);
    		executor.execute(ft2);
    
    		try {
    			countDown.await();
    	        System.out.println(poolSize+"个子线程已经执行完毕");
    	        System.out.println("主线程end use:" + (System.currentTimeMillis()-startTime));
    		} catch (InterruptedException e) {
    			// TODO Auto-generated catch block
    			e.printStackTrace();
    		}
    	}
    }

执行结果：

主线程start use:1546396254696  
pool-1-thread-2 user time 1011  
2个子线程已经执行完毕  
pool-1-thread-1 user time 1011  
主线程end use:1011

恩，完美，执行效率挺高的

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然后突然想到，如果poolSize设置成1呢，

import java.util.concurrent.Callable;
    import java.util.concurrent.CountDownLatch;
    import java.util.concurrent.ExecutorService;
    import java.util.concurrent.Executors;
    import java.util.concurrent.FutureTask;
    
    public class CountDownLatchTest {
    	private static int poolSize = 1;
    	private static CountDownLatch countDown = new CountDownLatch(poolSize);
    	
    	public static void main(String [] args) {
    
    		long startTime = System.currentTimeMillis();
    		System.out.println("主线程start use:" + startTime);
    		Callable<String> call1 = new Callable<String>() {
    		    @Override  
    		    public String call() throws Exception {  
    		        Thread.sleep(1000);  
    		        countDown.countDown();
    				System.out.println(Thread.currentThread().getName()+" user time "+(System.currentTimeMillis()-startTime));
    		        //return the thread name executing this callable task  
    		        return Thread.currentThread().getName();  
    		    } 
    			
    		};
    		Callable<String> call2 = new Callable<String>() {
    		    @Override  
    		    public String call() throws Exception {  
    		        Thread.sleep(1000); 
    		        countDown.countDown();  
    		        System.out.println(Thread.currentThread().getName()+" user time "+(System.currentTimeMillis()-startTime));
    		        //return the thread name executing this callable task  
    		        return Thread.currentThread().getName();  
    		    } 
    			
    		};
    		
    		FutureTask<String> ft1 = new FutureTask<String>(call1);
    		FutureTask<String> ft2 = new FutureTask<String>(call2);
    		
    		ExecutorService executor = Executors.newFixedThreadPool(poolSize);
    		
    		executor.execute(ft1);
    		executor.execute(ft2);
    
    		try {
    			countDown.await();
    	        System.out.println(poolSize+"个子线程已经执行完毕");
    	        System.out.println("主线程end use:" + (System.currentTimeMillis()-startTime));
    		} catch (InterruptedException e) {
    			// TODO Auto-generated catch block
    			e.printStackTrace();
    		}
    	}
    }

执行结果：

主线程start use:1546396057514  
pool-1-thread-1 user time 1011  
1个子线程已经执行完毕  
主线程end use:1011  
pool-1-thread-1 user time 2011

这，执行时间翻倍了啊，

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然后我又给加了个Callable,代码就不上了，直接放结果

执行结果：

主线程start use:1546396574511  
pool-1-thread-1 user time 1028  
1个子线程已经执行完毕  
主线程end use:1028  
pool-1-thread-1 user time 2029  
pool-1-thread-1 user time 3039

执行时间再次增加。。。。而且都是thread-1

**结论：多线程情况下，如果有m个线程，线程池大小为n,poolSize = m<n?m:n,会先创建poolSize个线程,然后创建剩余线程，并且线程编号重新开始排序**

是不是真的是这样呢，所有我又写了一个，1000个线程，但是每次会重新创建一次线程池，且size为1

import java.util.ArrayList;
    import java.util.List;
    import java.util.concurrent.Callable;
    import java.util.concurrent.CountDownLatch;
    import java.util.concurrent.ExecutorService;
    import java.util.concurrent.Executors;
    import java.util.concurrent.FutureTask;
    
    public class HighConcurrenceTest {
    	private static int poolSize = 1000;
    	private static CountDownLatch countDown = new CountDownLatch(poolSize);
    	public static void main(String [] args) {
    		List<FutureTask<String>> list = new ArrayList<FutureTask<String>>();
    		long startTime = System.currentTimeMillis();
    		System.out.println("主线程start use:" + startTime);
    		for(int i = 0;i<poolSize;i++) {
    			Callable<String> call = new Callable<String>() {
    			    @Override  
    			    public String call() throws Exception {  
    			        Thread.sleep(1000);  
    			        countDown.countDown();
    					System.out.println(Thread.currentThread().getName()+" user time "+(System.currentTimeMillis()-startTime));
    			        //return the thread name executing this callable task  
    			        return Thread.currentThread().getName();  
    			    } 
    				
    			};
    			FutureTask<String> future = new FutureTask<String>(call);
    			
    			ExecutorService executor = Executors.newFixedThreadPool(1);
    			executor.execute(future);
    			
    		}
    		
    		try {
    			countDown.await();
    			System.out.println("end use:" + (System.currentTimeMillis()-startTime));
    		} catch (InterruptedException e) {
    			// TODO Auto-generated catch block
    			e.printStackTrace();
    		}
    		
    	}
    }

执行结果：

主线程start use:1546398444063  
pool-84-thread-1 user time 1002  
pool-82-thread-1 user time 1002  
pool-86-thread-1 user time 1002

...

...  
pool-832-thread-1 user time 1050  
end use:1066  
pool-834-thread-1 user time 1050

...

pool-823-thread-1 user time 1050  
pool-826-thread-1 user time 1050  
pool-827-thread-1 user time 1050

上边的程序，虽然有1000个线程，由于每次往线程池放一个线程，所以执行时间并没有受影响

\--------------------------------------------------------------------------------------------------------------------------------

当然，实验不能就这样得结论，再测试一个

import java.util.ArrayList;
    import java.util.List;
    import java.util.concurrent.Callable;
    import java.util.concurrent.CountDownLatch;
    import java.util.concurrent.ExecutorService;
    import java.util.concurrent.Executors;
    import java.util.concurrent.FutureTask;
    
    public class HighConcurrenceTest {
    	private static int poolSize = 1000;
    	private static CountDownLatch countDown = new CountDownLatch(poolSize);
    	public static void main(String [] args) {
    		List<FutureTask<String>> list = new ArrayList<FutureTask<String>>();
    		long startTime = System.currentTimeMillis();
    		System.out.println("主线程start use:" + startTime);
    		for(int i = 0;i<poolSize;i++) {
    			Callable<String> call1 = new Callable<String>() {
    			    @Override  
    			    public String call() throws Exception {  
    			        Thread.sleep(1000);  
    			        countDown.countDown();
    					System.out.println(Thread.currentThread().getName()+" user time "+(System.currentTimeMillis()-startTime));
    			        //return the thread name executing this callable task  
    			        return Thread.currentThread().getName();  
    			    } 
    				
    			};
    			Callable<String> call2 = new Callable<String>() {
    			    @Override  
    			    public String call() throws Exception {  
    			        Thread.sleep(1000);  
    			        countDown.countDown();
    					System.out.println(Thread.currentThread().getName()+" user time "+(System.currentTimeMillis()-startTime));
    			        //return the thread name executing this callable task  
    			        return Thread.currentThread().getName();  
    			    } 
    				
    			};
    			FutureTask<String> future1 = new FutureTask<String>(call1);
    			FutureTask<String> future2 = new FutureTask<String>(call2);
    			
    			ExecutorService executor = Executors.newFixedThreadPool(1);
    			executor.execute(future1);
    			executor.execute(future2);
    			
    		}
    		
    		try {
    			countDown.await();
    			System.out.println("end use:" + (System.currentTimeMillis()-startTime));
    		} catch (InterruptedException e) {
    			// TODO Auto-generated catch block
    			e.printStackTrace();
    		}
    		
    	}
    }

执行结果：

主线程start use:1546398807709  
pool-3-thread-1 user time 1013  
pool-1-thread-1 user time 1013

...

pool-729-thread-1 user time 1080  
end use:1080  
pool-731-thread-1 user time 1080

...

pool-569-thread-1 user time 2112  
pool-570-thread-1 user time 2112  
pool-574-thread-1 user time 2112

\-----------------------------------------------------------------------------------------------------------------------------

ok,这些可以得结论啦，撒花撒花。。。。

**结论1：多线程情况下，如果有m个线程，线程池大小为n,poolSize = m<n?m:n,会先创建poolSize个线程,然后创建剩余线程，并且线程编号重新开始排序**

**结论2：多线程情况下，如果有m个线程，线程池大小为n,poolSize = m<n?m:n,会先创建poolSize个线程,然后创建剩余线程，并且线程编号重新开始排序，线程执行时间也会相应增加**

如有理解错误，欢迎讨论