Submit method of ThreadPoolExecutor thread pool

jdk1.7.0_79

## In the previous article "ThreadPoolExecutor thread The principle of the thread pool ThreadPoolExecutor and its execute method are mentioned in "Pool Principle and Its Execute Method". This article analyzes ThreadPoolExecutor#submit.

For the execution of a task, sometimes we don’t need it to return results, but there are times when we need its return execution results. For a thread, if it does not need to return a result, it can implement Runnable, and if it needs to execute the result, it can implement Callable. In the thread pool, execute also provides a task execution that does not need to return a result, and its submit method can be called for those that need to return a result.

Review the inheritance relationship of ThreadPoolExecutor.

　

Only the execute method is defined in the Executor interface, while the submit method is defined in the ExecutorService interface.

　

//ExecutorServicepublic interface ExecutorService extends Executor {
　　...
　　<T> Future<T> submit(Callable<T> task);
　　<T> Future<T> submit(Runnable task, T result);
　　<T> Future<T> submit(Runnable task);
　　...
}

　The submit method is implemented in its subclass AbstractExecutorService.

//AbstractExecutorServicepublic abstract class AbstractExecutorService implements ExecutorService {
　　...
　　public <T> Future<T> submit(Callable<T> task) {
　　　　if (task == null) throw new NullPointerException();
　　　　RunnableFuture<T> ftask = newTaskFor(task);
　　　　execute(ftask);
　　　　return ftask;
　　}
　　public <T> Future<T> submit(Runnable task, T result) {
　　　　if (task == null) throw new NullPointerException();
　　　　RunnableFuture<T> ftask = newTaskFor(task);
　　　　execute(ftask);
　　　　return ftask;
　　}
　　public Future<?> submit(Runnable task) {
　　　　if (task == null) throw new NullPointerExeption();
　　　　RunnableFuture<Void> ftask = newTaskFor(task, null);
　　　　execute(ftask);
　　　　return ftask; 
　　}
　　...
}

The submit method implemented in AbstractExecutorService is actually a template method that defines the algorithm skeleton of the submit method, and its execution is handed over to the subclass. (It can be seen that the template method pattern is widely used in many source codes. For the template method pattern, please refer to "Template Method Pattern")

Although the submit method can provide the return value of thread execution, only Callable is implemented There will be a return value, but the thread that implements Runnable has no return value. That is to say, among the above three methods, submit(Callable task) can get its return value, submit(Runnable task, T result) can indirectly obtain the return value of the thread through the incoming carrier result or to be precise, hand it over to the thread for processing. The last method, submit (Runnable task), has no return value. Even if it gets its return value, it will be null. .

Below are three examples to get a feel for the submit method.

　submit(Callable task)

package com.threadpoolexecutor;import java.util.concurrent.*;/**
 * ThreadPoolExecutor#sumit(Callable<T> task)
 * Created by yulinfeng on 6/17/17. */public class Sumit1 {public static void main(String[] args) throws ExecutionException, InterruptedException {
        Callable<String> callable = new Callable<String>() {public String call() throws Exception {
                System.out.println("This is ThreadPoolExetor#submit(Callable<T> task) method.");return "result";
            }
        };

        ExecutorService executor = Executors.newSingleThreadExecutor();
        Future<String> future = executor.submit(callable);
        System.out.println(future.get());
    }
}

　submit(Runnable task, T result)

package com.threadpoolexecutor;import java.util.concurrent.ExecutionException;import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;import java.util.concurrent.Future;/**
 * ThreadPoolExecutor#submit(Runnable task, T result)
 * Created by yulinfeng on 6/17/17. */public class Submit2 {public static void main(String[] args) throws ExecutionException, InterruptedException {

        ExecutorService executor = Executors.newSingleThreadExecutor();
        Data data = new Data();
        Future<Data> future = executor.submit(new Task(data), data);
        System.out.println(future.get().getName());
    }
}class Data {
    String name;public String getName() {return name;
    }public void setName(String name) {this.name = name;
    }
}class Task implements Runnable {
    Data data;public Task(Data data) {this.data = data;
    }public void run() {
        System.out.println("This is ThreadPoolExetor#submit(Runnable task, T result) method.");
        data.setName("kevin");
    }
}

　submit(Runnable task)

package com.threadpoolexecutor;import java.util.concurrent.ExecutionException;import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;import java.util.concurrent.Future;/**
 * ThreadPoolExecutor#sumit(Runnable runnables)
 * Created by yulinfeng on 6/17/17. */public class Submit {public static void main(String[] args) throws ExecutionException, InterruptedException {
        Runnable runnable = new Runnable() {public void run() {
                System.out.println("This is ThreadPoolExetor#submit(Runnable runnable) method.");
            }
        };

        ExecutorService executor = Executors.newSingleThreadExecutor();
        Future future = executor.submit(runnable);
        System.out.println(future.get());
    }
}

From the above example, we can see that when calling submit(Runnable runnable), its defined type is not required. That is to say, although it is defined as a generic method in ExecutorService, it is not generic in AbstractExecutorService. method because it has no return value. (For the differences between Object, T, and ?, please refer to "The Difference between Object, T (Generics), and ? in Java").

As you can see from the source code above, these three methods are almost the same. The key lies in:

RunnableFuture<T> ftask = newTaskFor(task);
execute(ftask);

How it passes a task as a parameter to What about newTaskFor, then calling the execute method, and finally returning ftask?

//AbstractExecutorService#newTaskForprotected <T> RunnableFuture<T> newTaskFor(Callable<T> callable) {
　　return new FutureTask<T>(callable);
}
　　protected <T> RunnableFuture<T> newTaskFor(Runnable runnable, T value) {
　　return new FutureTask<T>(runnable, value);
}

It seems that a FutureTask instance is returned, and FutureTask implements the Future and Runnable interfaces. The Future interface is an implementation of the Java thread Future mode, which can be used for asynchronous calculations. Implementing the Runnable interface means that it can be executed as a thread. FutureTask implements these two interfaces, which means that it represents the result of asynchronous calculation and can be handed over to the Executor as a thread for execution. FutureTask will be analyzed separately in the next chapter. Therefore, this article's analysis of the submit method of the thread pool ThreadPoolExecutor thread pool is not complete. You must understand the Future mode of Java threads - "14. Future Mode in Java".

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