线程池ThreadPoolExecutor的submit方法

   <T> Future<T> submit(Callable<T> task);

   <T> Future<T> submit(Runnable task, T result);

    Future<?> submit(Runnable task);

三个重载方法，第一个很好理解，传入callable，本身call函数就有一个返回值，可以正常取得。

第二个第三个就不清楚怎么可以取到返回值了，因为Runnable本身的run方法也没有返回值，为什么要这么做，如果没有返回值，直接用execute方法就好，为什么还要下面这几个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) {

}
}
}

输出：

hehe
null

submit提交的任务，正常执行，返回的是null，不清楚有什么意义

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

任务里抛出异常，返回值可以get到该异常，继续抛出，不懂。

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();
}

}

输出：

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) {

}
}
}

输出：

how
he

结论：终于还是没搞懂，这两个函数的使用场景是什么。
觉得应该是：

execute提交runnable执行就好

submit提交callable,取得返回值就好

为什么要submit提交runnable呢