Runtime exec()调用shell脚本

有时，我们需要在java程序中调用外部程序，我们可用通过Runtime.exec()调用来完成。

The class java.lang.Runtime features a static method called getRuntime() , which retrieves the current Java Runtime Environment. That is the only way to obtain a reference to the Runtime object. With that reference, you can run external programs by invoking the Runtime class's exec() method. Developers often call this method to launch a browser for displaying a help page in HTML.

exec()有四个重载版本

There are four overloaded versions of the exec() command:

public Process exec(String command);
public Process exec(String [] cmdArray);
public Process exec(String command, String [] envp);
public Process exec(String [] cmdArray, String [] envp);
For each of these methods, a command -- and possibly a set of arguments -- is passed to an operating-system-specific function call. This subsequently creates an operating-system-specific process (a running program) with a reference to a Process class returned to the Java VM. The Process class is an abstract class, because a specific subclass of Process exists for each operating system.

You can pass three possible input parameters into these methods:

A single string that represents both the program to execute and any arguments to that program
An array of strings that separate the program from its arguments
An array of environment variables
Pass in the environment variables in the form name=value . If you use the version of exec() with a single string for both the program and its arguments, note that the string is parsed using white space as the delimiter via the StringTokenizer class.

注意事项：

1.  当调用的外部命令中包含重定向(<、>)，管道( | ) 命令时，exec(String command)的版本不能正确解析重定向、管道操作符。所以需要使用exec(String [] cmdArray)。
   如，echo "hello world" > /home/admin/newFile.txt
       ls -e | grep java
   需要使用如下的调用方式
      String []cmdArray = new String[]{ "/bin/sh", "-c", "ls -e | grep java"};
       Runtime.getRuntime().exec(cmdArray);
2.  
永远要在调用waitFor()方法之前读取数据流
永远要先从标准错误流中读取，然后再读取标准输出流
The next version of Savant is going to focus heavily on the stand-alone runtime anｄ support for dialects anｄ plugins. Supporting all that is largely handled by using a simple executor framework I wrote around Java 1.4 anｄ lower’s Runtime.exec method. A few things to keep in mind when using this:

Always read from the streams prior to calling waitFor. Otherwise you could end up waiting forever on Windows anｄ other OS platforms whose I/O buffers can’t store enough from standard out anｄ standard error to ensure the program has finished. These platforms will pause the execution of whatever is running until something reads the buffered content from standard out anｄ standard error. I would imagine all platforms suffer from this, but some platforms have larger buffers than others. Needless to say, always read from the streams first.
Always read from standard error first. I ran across a bug where some OS platforms will always open standard out, but never close it. What this means is that if you read from standard out first anｄ the process only writes to standard error, you’ll hang forever waiting to read. If you read from standard error first, you’ll always be okay on these platforms because the OS seems to shutdown standard error. I think however, that the best way to handle all cases is to check both standard error anｄ standard out for readiness anｄ only read from them if they have something to offer. The downside I could see here is that error isn’t ready, but eventually will be.
Because some native platforms only provide limited buffer size for standard input and output streams, failure to promptly write the input stream or read the output stream of the subprocess may cause the subprocess to block, and even deadlock.

在exec()后 立即调用waitFor()会导致进程挂起。