我正在尝试执行一个新进程并从Java中读取其输入流。我已成功使用Runtime.getRuntime()。exec(String)来启动和接收来自多个进程的输入。但是,当我尝试在某些其他进程上使用exec时,输入流的read方法会阻塞,并且看起来没有输入。对于某些进程,可能导致输入流为空的原因是什么?具体来说,我想知道为什么bash.exe没有输出任何东西。
我编写了一个JUnit测试用例来演示这个问题:
import java.io.IOException;
import java.io.InputStream;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import junit.framework.TestCase;
public class TestExec extends TestCase {
public void testExec() throws IOException {
List<InputPrinter> threads = new ArrayList<InputPrinter>();
// Create a process for each of the commands and make sure that
// it outputs at least one line to its input stream.
threads.add(testExec("cmd"));
threads.add(testExec("java"));
threads.add(testExec("C:/cygwin/bin/vim-nox.exe"));
// These bottom two fail, even though executing these
// commands in cmd.exe results in immediate output
threads.add(testExec("javac"));
threads.add(testExec("C:/cygwin/bin/bash.exe"));
// Give the threads a second to execute
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
fail();
}
// Test that each command had input to read
for(InputPrinter ip : threads) {
assertTrue(ip.command + " has not read any input", ip.hasRead);
}
}
// Starts a process for the given command and returns an
// InputPrinter that can be used to check if the process
// has had an input to read.
public InputPrinter testExec(String command) throws IOException {
Process proc = Runtime.getRuntime().exec(command);
InputStream in = proc.getInputStream();
InputPrinter ip = new InputPrinter(in, command);
new Thread(ip).start();
return ip;
}
// Simple Runnable to read from an InputStream. hasRead will be
// true if at least one input has been read from the stream
private class InputPrinter implements Runnable {
InputStream in;
String command;
boolean hasRead;
public InputPrinter(InputStream in, String command) {
this.in = in;
this.command = command;
this.hasRead = false;
}
// Loop indefinitely while printing any received input
public void run() {
try {
final byte[] b = new byte[1024];
while (true) {
int n = in.read(b);
if (n > 0) {
System.out.print(new String(Arrays.copyOf(b, n)));
hasRead = true;
}
}
} catch (IOException e) {
e.printStackTrace();
fail();
}
}
}
}
编辑:
据我所知,如果程序没有使用stdout或stderr,我在Windows命令提示符下看不到任何内容。当我启动bash进程时,我期待看到的是“bash-3.2 $”,当我打开命令提示符并运行“bash.exe”时,我看到了同样的事情:
Microsoft Windows [Version 6.1.7600]
Copyright (c) 2009 Microsoft Corporation. All rights reserved.
C:\cygwin\bin>bash.exe
bash-3.2$
答案 0 :(得分:2)
无论Java如何,据我所知,只有当它作为脚本运行时,才能将输出(或输入)从/向bash传输,而不是在它作为交互式shell运行时(在这种情况下你只能通过)它的cmd参数)。
换句话说,当您在注释中提到从cmd运行bash时,您会看到输出,但它包含在bash进程中,而不会输出bash发送回父cmd进程。
关于javac进程,它实际上是将输出发送到错误流。尝试从cmd javac 1>null
和javac 2>null
开始运行,您将看到不同之处
你看过api here了吗?您可以尝试使用ProcessBuilder并将错误流重定向回主输入流,以这种方式处理流程会更容易。
答案 1 :(得分:2)
一个进程通常不仅有一个而是两个与之关联的输出流。这些是:
Javac写入stderr,而不是stdout,因此你不会读取它的输出。
因为不方便阅读它们是不方便的(几年前,我不得不为此写一个额外的线程),他们为系统进程引入了一个新的API,即ProcessBuilder,它允许将stderr重定向到stdout
只需替换
行 Process proc = Runtime.getRuntime().exec(command);
InputStream in = proc.getInputStream();
与
ProcessBuilder pb = new ProcessBuilder(command);
pb.redirectErrorStream(true);
Process proc = pb.start();
,添加所需的导入,测试成功:)。