Future<?> submit(Runnable task);

三個過載方法，第一個很好理解，傳入callable，本身call函式就有一個返回值，可以正常取得。

第二個第三個就不清楚怎麼可以取到返回值了，因為Runnable本身的run方法也沒有返回值，為什麼要這麼做，如果沒有返回值，直接用execute方法就好，為什麼還要下面這幾個submit方法呢。

package thread.pool;

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;

public class PoolTest {
    public static void main(String[] args) {
		ExecutorService executor = Executors.newFixedThreadPool(3);
		Future<?> submit = executor.submit(new Runnable(){

			@Override
			public void run() {
				System.out.println("hehe");				
			}			
		});
		try {
			System.out.println(submit.get());
		} catch (Exception e) {
			
		}
	}
}
 

hehe
null

package thread.pool;

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;

public class PoolTest {
    public static void main(String[] args) {
		ExecutorService executor = Executors.newFixedThreadPool(3);
		Future<?> submit = executor.submit(new Runnable(){

			@Override
			public void run() {
				System.out.println("how");
				throw new RuntimeException("hehe");				
			}			
		});
		try {
			Object obj = submit.get();
			System.out.println(obj);
		} catch (Exception e) {
			
			System.out.println(e.getMessage() + "  exception");
			
		}
	}
}
 

how
java.lang.RuntimeException: hehe  exception

package thread.pool;

import java.util.concurrent.Callable;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;
import java.util.concurrent.FutureTask;

public class PoolTest {
    public static void main(String[] args) {
		ExecutorService executor = Executors.newFixedThreadPool(3);
		FutureTask<String> ft = new FutureTask<String>(new Callable<String>(){

			@Override
			public String call() throws Exception {
				
				return "hehe";
			}
			
		});
		Future<?> submit = executor.submit(ft);
		try {
			;
			System.out.println("1" + submit.get());
			System.out.println("2" + ft.get());
		} catch (Exception e) {
			
			System.out.println(e.getMessage() + "  exception");
			
		}
		executor.shutdown();
	}
   
}
 

1null
2hehe

package thread.pool;

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;

public class PoolTest {
	static String aa = "he";
    public static void main(String[] args) {
		ExecutorService executor = Executors.newFixedThreadPool(3);
		
		Future<?> submit = executor.submit(new Runnable(){

			@Override
			public void run() {
				System.out.println("how");
				aa = aa + "he";			
			}			
		},aa);
		try {
			System.out.println(submit.get());
		} catch (Exception e) {
			
		}
	}
}

how
he

覺得應該是：

execute提交runnable執行就好

submit提交callable,取得返回值就好

為什麼要submit提交runnable呢