java - 为什么这个程序创建的线程比可能的多？

Question

这适用于配置单元查询中的自定义 UDTF，CreateLogTable是我用作测试临时对象的 UDTF 类。我正在为每个 file to be downloaded from Amazon S3 创建一个线程并等待另一个线程变得可用，然后再将另一个文件分配给该线程。

主要测试逻辑:

CreateLogTable CLT = new CreateLogTable();

int numThreads = 2;
int index = 0;
DownloadFileThread[] dlThreads = new DownloadFileThread[numThreads];
for (S3ObjectSummary oSummary : bucketKeys.getObjectSummaries()) {
    while (dlThreads[index] != null && dlThreads[index].isAlive()) {
        index += 1;
        index = index % numThreads;
    }
    dlThreads[index] = new DownloadFileThread(CLT , getBucket(oSummary.getBucketName() + "/"
                    + oSummary.getKey()), getFile(oSummary.getKey()), index);
    dlThreads[index].start();
    index += 1;
    index = index % numThreads;
}

线程类(run()方法):

try {
    System.out.println("Creating thread " + this.threadnum);
    this.fileObj = this.S3CLIENT.getObject(new GetObjectRequest(this.filePath, this.fileName));
    this.fileIn = new Scanner(new GZIPInputStream(this.fileObj.getObjectContent()));
    while (this.fileIn.hasNext()) {         
        this.parent.forwardToTable(fileIn.nextLine());
    }
    System.out.println("Finished " + this.threadnum);
} catch (Throwable e) {
    System.out.println("Downloading of " + this.fileName + " failed.");
}

创建线程之前的 while 循环应该一直循环，直到找到 null thread或dead thread直到它退出循环，在这种情况下 new thread将被创建并启动。由于我将日志记录到控制台，因此我能够观察此过程，但输出是意外的:

Creating thread 0
Creating thread 1
Creating thread 0
Creating thread 1
Creating thread 0
Creating thread 1
Creating thread 0
...
Creating thread 1
Creating thread 0
Creating thread 1
Finished 0
Finished 1
Finished 1
Finished 0
Finished 1
Finished 1
...
Finished 0
Finished 1
Finished 0
Finished 1

上面只是输出的前几行。问题在于，在任何线程完成其任务之前，会创建两个以上的线程。

为什么会发生这种情况以及如何解决这个问题？

Answer 1

我将您的代码简化为这个测试用例:

public class ThreadTest {
    private static class SleepThread extends Thread {
        private final int index;
        SleepThread(int ii) { index = ii; }

        @Override
        public void run() {
            System.out.println("Creating thread " + this.index);
            try {
                Thread.sleep(5_000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("Finished " + this.index);
        }
    }

    public static void main(String[] args) {
        int numThreads = 2;
        int index = 0;
        SleepThread[] dlThreads = new SleepThread[numThreads];
        for (int ii = 0; ii < 10; ++ii) {
            while (dlThreads[index] != null && dlThreads[index].isAlive()) {
                index += 1;
                index = index % numThreads;
            }
            dlThreads[index] = new SleepThread(index);
            dlThreads[index].start();
            index += 1;
            index = index % numThreads;
        }
    }
}

使用 Sun JDK 1.7.0_75，运行它会产生您期望的结果 - 两个线程启动，它们在五秒后退出，另外两个线程启动，依此类推。

我怀疑的下一件事是，您的 JVM 的 Thread.isAlive() 实现在线程启动后不会立即返回 true，尽管这似乎与文档相反Thread 类。