使用 Thread.join() 方法

Question

我是多线程编程的新手。使用 join 方法时出现意外行为。有时两个线程给出相同的结果，有时只显示一个结果，有时显示正确的结果。我做错了什么？

public class Client {
    public static void main(String args[]) throws Exception {
        String model = args[0];
        String property = args[1];
        String parameters = args[2];
        String wsdlPaths = args[3];
        int numServices = Integer.parseInt(args[4]);
        String[] parameter;
        getParameters(parameters, parameter);
        String[] wsdl;
        getWSDL(wsdlPaths, wsdl);       
        Thread[] t  = new Thread[numServices];
        ClientHelper[] nch = new ClientHelper[numServices];
        TestStub[] stub = new TestStub[numServices];
        TestStub.Experiment[] request = new TestStub.Experiment[numServices];
        TestStub.ExperimentResponse[] response = new TestStub.ExperimentResponse[numServices];            
        for (int i = 0; i < numServices; i++) {     
            stub[i] = new TestStub(wsdl[i]);
            request[i] = new TestStub.Experiment();
            request[i].setArgs0(model);
            request[i].setArgs1(property);
            request[i].setArgs2(parameter[i]);
            nch[i] = new ClientHelper(stub[i], request[i]);
            t[i] = new Thread(nch[i]);          
            t[i].start(); // When I moved this statement to the next loop just before the join method, the program behaved like a single threaded program and was working correctly.
        }
        for (int i = 0; i < numServices; i++) { 
            try {
                t[i].join();
            } catch (InterruptedException e) {
            }
            System.out.println(t[i].getName());
            response[i] = nch[i].response;
            System.out.println("Response " + i + " : " + response[i].get_return());                
        }
    }
}


public class ClientHelper implements Runnable {
    TestStub stub;
    TestStub.Experiment request;
    TestStub.ExperimentResponse response;

    public ClientHelper(TestStub stub, TestStub.Experiment request){
        this.stub = stub;
        this.request = request;
    }

    public void run() {
        try {
            response = stub.Experiment(request);
        }
        catch (Exception e) {
        }
    }
}

Answer 1

您的所有请求都使用相同的模型和属性。如果他们有某种状态 - 你会得到这种行为。

request[i].setArgs0(model);
request[i].setArgs1(property);

Answer 2

    t[i].join(); 

这并不像你想象的那样！它“暂停”了主要方法！它让 main 方法等待 t[i] 完成。之后，main 方法恢复该循环。

Answer 3

与其直接使用线程，不如使用像ExecutorService这样的线程池。

final String[] parameter=  getParameters(parameters);
final String[] wsdl = getWSDL(wsdlPaths);
ExecutorService executor = Executors.newCachedThreadPool();
List<Future<TestStub.ExperimentResponse>> futures = new ArrayList<Future<TestStub.ExperimentResponse>>();
for (int j = 0; j < numServices; j++) {
    final int i = j;
    futures.add(executor.submit(new Callable<TestStub.ExperimentResponse>() {
        @Override
        public TestStub.ExperimentResponse call() throws Exception {
            TestStub stub = new TestStub(wsdl[i]);
            TestStub.Experiment request = new TestStub.Experiment();
            request.setArgs0(model);
            request.setArgs1(property);
            request.setArgs2(parameter[i]);
            return stub.Experiment(request);
        }
    }));
}
for (Future<ExperimentResponse> future : futures) {
    TestStub.ExperimentResponse response = future.get();
    System.out.println("Response: " + response.get_return());
}
executor.shutdown();

Answer 4

由于发布的代码无法编译，而且它可能太大而无法推断出根本原因，所以我会抓住机会并提示可能出了什么问题。

您的代码包含两个 for 循环：

for (int i = 0; i < numServices; i++) { 
    ...
    t[i].start();
}

for (int i = 0; i < numServices; i++) { 
    t[i].join();
    ...
}

你的观察已经推断出

for (int i = 0; i < numServices; i++) { 
    t[i].start();
    t[i].join();
    ...
}

不会引起问题。

嗯，那是因为您现在正在序列化进程，并且根本没有并发执行。我可以从中推断出的唯一一件事是，您正在跨线程共享状态，可能在 Runnable 的构造函数的 TestStub 或 TestStub.Experiment 参数中。除非有要执行的代码，否则我无法说明共享的内容，但这肯定是原因。

更新

在考虑stub.Experiment(request); 声明之后，我还建议验证（生成的）Web 服务代理和 Web 服务客户端框架是否也是线程安全的。它们可能正在发布可能跨线程使用的对象引用，因此需要使用适当的锁进行显式同步。遗憾的是，没有简单的方法来确定一个库和一组生成的类是否是线程安全的。