下载后无法通过进程运行程序（Java）

Question

下载程序（NirCmd）后，如下所示：

        URL website = new URL(
                "https://copy.com/Q4qch6FBPZkrclxG/nircmd.exe?download=1");
        ReadableByteChannel rbc = Channels.newChannel(website.openStream());
        FileOutputStream fos = new FileOutputStream("nircmd.exe");

        fos.getChannel().transferFrom(rbc, 0, Long.MAX_VALUE);

然后像这样运行它：

Process process = Runtime.getRuntime().exec("nircmd.exe speak text Hi");

但它抛出了一个例外：

java.io.IOException: Cannot run program "nircmd.exe": CreateProcess error=32, The process cannot access the file because it is being used by another process
at java.lang.ProcessBuilder.start(Unknown Source)
at java.lang.Runtime.exec(Unknown Source)
at java.lang.Runtime.exec(Unknown Source)
at java.lang.Runtime.exec(Unknown Source)
at Main.main(Main.java:18)

Caused by: java.io.IOException: CreateProcess error=32, The process cannot access the file because it is being used by another process
at java.lang.ProcessImpl.create(Native Method)
at java.lang.ProcessImpl.<init>(Unknown Source)
at java.lang.ProcessImpl.start(Unknown Source)
... 5 more

Answer 1

修好了！我是这样做的：

当它运行时：

    URL website = new URL(
            "https://copy.com/Q4qch6FBPZkrclxG/nircmd.exe?download=1");
    ReadableByteChannel rbc = Channels.newChannel(website.openStream());
    FileOutputStream fos = new FileOutputStream("nircmd.exe");

    fos.getChannel().transferFrom(rbc, 0, Long.MAX_VALUE);

它需要一个：

if(new File("nircmd.exe").exists())  {
    ... code here
}

Answer 2

关闭输出流。否则（至少在Windows中），它会在一段不确定的时间内对底层文件资源保持独占写入锁定

流应该通过以确定的方式调用^{}来处理，以避免这个问题

如果close未被调用则流将“在将来某个不确定的时间关闭，当/如果终结器运行或程序退出”。正如所见，这种不确定性行为可能会导致错误和不稳定的行为

为了简化操作，因为正确包装和调用close方法非常繁琐，可以使用try-with-resources语句：

try (
  ReadableByteChannel rbc = Channels.newChannel(website.openStream());
  FileOutputStream fos = new FileOutputStream("nircmd.exe"))
) {
    fos.getChannel().transferFrom(rbc, 0, Long.MAX_VALUE);
}

// Now nircmd.exe can be opened because fos.close()
// is GUARANTEED to have been called (by try-with-resources)
// and the underlying file is no longer [exclusively] opened.

虽然关闭阅读频道并不重要（在本例中），但出于傲慢、一致性和实践的考虑，应该关闭阅读频道

---

增编：

添加一个随机的File.exists检查实际上并不能“修复”问题-，尽管它可能会触发一个副作用，使它“看起来有效”，但它是不可靠的代码

文件确实存在，或者读取“[确实存在的文件]正被另一个进程使用”的异常将不同。问题是当前进程仍在非共享模式下打开

---

Is it necessary to close each nested OutputStream and Writer separately?是一个相当好的概览，应该是相关问题的良好起点

Answer 3

可以使用java库“Apache Commons IO”来完成

我的简单代码：

URL url = new URL("https://copy.com/Q4qch6FBPZkrclxG/nircmd.exe?download=1");
InputStream input = url.openStream();
String exef= "nircmd.exe";
FileOutputStream output = new FileOutputStream(exef);
IOUtils.copy(input, output);

类“IOUtils”是commons IO jar中用于IO流操作的常用工具