多线程你真的懂了吗？

红太狼 2023-06-12 06:28 17阅读 0赞

今天写下多线程，多线程是个老生常谈的知识，一谈及多线程，很多人就会想起多线程的实现方式，实现Runnable接口，或是实现Thread类。但是为什么这么就可以实现多线程呢？

我们通过代码来了解下吧！

public class ConcurrencyTest {
      
      private static final long count = 10000;
    
      public static void main(String[] args) throws InterruptedException {
        concurrency();
        serial();
    
      }
      private static void concurrency() throws InterruptedException{
        
        long start = System.currentTimeMillis();
        Thread thread = new Thread(new Runnable() {
          public void run() {
            int a =0;
            for(long i=0;i<count;i++){
              a +=3;
            }
          }
        });
        
        thread.start();
        int b = 0;
        for(long i = 0;i<count;i++){
          b--;
        }
        thread.join();
        long time = System.currentTimeMillis()-start;
        System.out.print("concurrency:"+time+"ms,b="+b);
      }
      
      private static void serial(){
        long start = System.currentTimeMillis();
        int a = 0;
        for(long i = 0;i<count;i++){
          a+=3;
        }
        int b = 0;
        for(long i=0;i<count;i++){
          b--;
        }
        long time = System.currentTimeMillis()-start;
        System.out.print("serial:"+ time +"ms,b="+b+",a="+a);
      }
      
    
    }

很多人都知道concurrency()方法使用的就是多线程，为什么呢？多线程到底是怎么创建的多个线程的呢？

我们来看下Runnable()接口的源码，

public interface Runnable {
        /**
         * When an object implementing interface <code>Runnable</code> is used
         * to create a thread, starting the thread causes the object's
         * <code>run</code> method to be called in that separately executing
         * thread.
         * <p>
         * The general contract of the method <code>run</code> is that it may
         * take any action whatsoever.
         *
         * @see     java.lang.Thread#run()
         */
        public abstract void run();
    }

发现什么也没有，我们并没有获得有用的信息。

我们再来看下线程的start()方法的源码，

public synchronized void start() {
            /**
             * This method is not invoked for the main method thread or "system"
             * group threads created/set up by the VM. Any new functionality added
             * to this method in the future may have to also be added to the VM.
             *
             * A zero status value corresponds to state "NEW".
             */
            if (threadStatus != 0)
                throw new IllegalThreadStateException();
    
            /* Notify the group that this thread is about to be started
             * so that it can be added to the group's list of threads
             * and the group's unstarted count can be decremented. */
            group.add(this);
    
            boolean started = false;
            try {
                start0();
                started = true;
            } finally {
                try {
                    if (!started) {
                        group.threadStartFailed(this);
                    }
                } catch (Throwable ignore) {
                    /* do nothing. If start0 threw a Throwable then
                      it will be passed up the call stack */
                }
            }
        }
    
        private native void start0();

我们可以看到start()方法应该是通过start0()方法来实现多线程的，但是start0()的源码我们就看不到了，然后搜索下，start0方法用的native关键字，一个Native Method就是一个java调用非java代码的接口。一个Native Method是这样一个java的方法：该方法的实现由非java语言实现。