Java多线程实现原理

超、凢脫俗 2022-08-31 13:18 109阅读 0赞

**1.多线程的创建方式Thread与Runnable方式**

new Thread() {
                public void run() {
                    System.out.println("直接使用Thread创建");
                };
            }.start();
    
            new Thread(new Runnable() {
                @Override
                public void run() {
                    System.out.println("传入Runnable创建");
                }
            }).start();

原理：

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2JhaWR1XzMyNDkyODQ1_size_16_color_FFFFFF_t_70][]

**2.Callable方式**

FutureTask<String> task1 = new FutureTask(new Callable(){
                public String call(){
                    return "线程实现方式";
                }
            });
            Thread thread = new Thread(task1);
            thread.start();
            try {
                System.out.println(task1.get());
            } catch (InterruptedException e) {
                e.printStackTrace();
            } catch (ExecutionException e) {
                e.printStackTrace();
            }

**a.使用线程实现方式**

主要是将Callable的实例对象通过FutureTask包装了一下，可以看下FutureTask中构造方法的源码

FutureTask 实现RunnableFuture接口，而RunnableFuture接口继承Runnable, Future接口

因此FutureTask可以包装的Callable的实例中的call()在run()中执行，所以这种方式就等于将Runnable实现的子类实例放入到Thread的构造方法中，子类的run()方法正好是重写里面调用call();

public void run() {
            if (state != NEW ||
                !UNSAFE.compareAndSwapObject(this, runnerOffset,
                                             null, Thread.currentThread()))
                return;
            try {
                Callable<V> c = callable;
                if (c != null && state == NEW) {
                    V result;
                    boolean ran;
                    try {
                        result = c.call();
                        ran = true;
                    } catch (Throwable ex) {
                        result = null;
                        ran = false;
                        setException(ex);
                    }
                    if (ran)
                        set(result);
                }
            } finally {
                // runner must be non-null until state is settled to
                // prevent concurrent calls to run()
                runner = null;
                // state must be re-read after nulling runner to prevent
                // leaked interrupts
                int s = state;
                if (s >= INTERRUPTING)
                    handlePossibleCancellationInterrupt(s);
            }
        }

**另外**，FutureTask的构造方法传入的参数除了Callable之外也支持Runnable，它的操作主要是将Runnable实现的子类通过RunnableAdapter转化为Callable，源码如下

public FutureTask(Runnable runnable, V result) {
            this.callable = Executors.callable(runnable, result);
            this.state = NEW;       // ensure visibility of callable
        }
    
    
        public static <T> Callable<T> callable(Runnable task, T result) {
            if (task == null)
                throw new NullPointerException();
            return new RunnableAdapter<T>(task, result);
        }

**b.使用线程池方式实现**

ThreadPoolExecutor threadPool =   (ThreadPoolExecutor)Executors.newFixedThreadPool (10);
            Callable callable = new Callable() {
                @Override
                public Object call() throws Exception {
                    return "线程池实现方式";
                }
            };
            FutureTask<String> task = new FutureTask<>(callable);
            threadPool.submit(task);
            try {
                System.out.println(task.get());;
            } catch (InterruptedException e) {
                e.printStackTrace();
            } catch (ExecutionException e) {
                e.printStackTrace();
            }

submit()传入的是Runnable, FutureTask继承的就是Runnable，submit里面的execute的运行是根据newFixedThreadPool中的execute()运行。下面为submit()的源码

public Future<?> submit(Runnable task) {
            if (task == null) throw new NullPointerException();
            RunnableFuture<Void> ftask = newTaskFor(task, null);
            execute(ftask);
            return ftask;
        }

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