至少有两个问题。

首先，关闭ScannerandPrintWriter也将关闭它们的不死流（在这种情况下Scanner是从标准输入读取的）。这是不可取的，因为，首先，您永远不想关闭 stdin 流，其次，该过程可能尚未完成对流的处理，但更重要的是，我会非常小心关闭不是您自己创建的流，只是说.

第二个问题是，您试图在SPQ实际有时间处理输出之前读取（流的）输出，例如，如果我调整您的代码并使用...

import java.io.IOException;
import java.io.InputStream;
import java.util.LinkedList;
import java.util.Queue;

public final class Main {
    public static void main(String[] args) throws IOException, InterruptedException {
        new Main();
    }

    public Main() throws IOException, InterruptedException {
        ProcessBuilder pb = new ProcessBuilder("bash", "-c", "ls");
        pb.redirectErrorStream(true);
        System.out.println(">> Start process");
        Process p = pb.start();
        Queue<String> stdout = new LinkedList<>();
        Thread thread = new Thread(new SPQ(p.getInputStream(), stdout));
        System.out.println(">> Start thread");
        thread.start();
        System.out.println(">> Waiting for process to exit");
        p.waitFor();
        System.out.println("<< Process has exited");
        System.out.println(stdout);
    }

    class SPQ implements Runnable {
        private Queue<String> queue;
        private InputStream istrm;

        public SPQ(InputStream istrm, Queue<String> queue) {
            this.istrm = istrm;
            this.queue = queue;
        }

        public void run() {
            System.out.println(">> Started reading stream");
            try {
                final byte[] buffer = new byte[1024];
                for (int length = 0; (length = istrm.read(buffer)) != -1;) {
                    queue.add(new String(buffer, 0, length)); // store output in queue
                }
            } catch (Exception e) {
                e.printStackTrace();
            }
            System.out.println("<< Done reading stream");
        }
    }
}

它会输出...

>> Start process
>> Start thread
>> Waiting for process to exit
<< Process has exited
>> Started reading stream
[]
<< Done reading stream

如您所见，我已经Queue在线程完成处理流之前打印了内容。

如果我在打印thread.join();后添加p.waitFor();...

>> Start process
>> Start thread
>> Waiting for process to exit
>> Started reading stream
<< Done reading stream
<< Process has exited
[contents
of
my
working
directory
which
is
not
very
interesting]

可运行的示例...

import java.io.IOException;
import java.io.InputStream;
import java.util.LinkedList;
import java.util.Queue;

public final class Main {
    public static void main(String[] args) throws IOException, InterruptedException {
        new Main();
    }

    public Main() throws IOException, InterruptedException {
        ProcessBuilder pb = new ProcessBuilder("bash", "-c", "ls");
        pb.redirectErrorStream(true);
        System.out.println(">> Start process");
        Process p = pb.start();
        Queue<String> stdout = new LinkedList<>();
        Thread thread = new Thread(new SPQ(p.getInputStream(), stdout));
        System.out.println(">> Start thread");
        thread.start();
        System.out.println(">> Waiting for process to exit");
        p.waitFor();
        thread.join();
        System.out.println("<< Process has exited");
        System.out.println(stdout);
    }

    class SPQ implements Runnable {
        private Queue<String> queue;
        private InputStream istrm;

        public SPQ(InputStream istrm, Queue<String> queue) {
            this.istrm = istrm;
            this.queue = queue;
        }

        public void run() {
            System.out.println(">> Started reading stream");
            try {
                final byte[] buffer = new byte[1024];
                for (int length = 0; (length = istrm.read(buffer)) != -1;) {
                    queue.add(new String(buffer, 0, length)); // store output in queue
                }
            } catch (Exception e) {
                e.printStackTrace();
            }
            System.out.println("<< Done reading stream");
        }
    }
}

请注意 - 我没有运行 Windows，所以我无法测试Powershell并且我已经完成了我能想到的最小示例，所以我没有将内容写入流，但从概念上讲，这解决了你似乎是的问题有