Runtime.getRuntime().exec()执行JVM之外的程序：常见的几种陷阱
前言
日常java开发中，有时需要通过java运行其它应用功程序，比如shell命令等。jdk的Runtime类提供了这样的方法。首先来看Runtime类的文档, 从文档中可以看出，每个java程序只会有一个Runtime实例，显然这是一个单例模式。
/**
* Every Java application has a single instance of class
* <code>Runtime</code> that allows the application to interface with
* the environment in which the application is running. The current
* runtime can be obtained from the <code>getRuntime</code> method.
* <p>
* An application cannot create its own instance of this class.
*/
public class Runtime {
private static Runtime currentRuntime = new Runtime();
/**
* Returns the runtime object associated with the current Java application.
* Most of the methods of class <code>Runtime</code> are instance
* methods and must be invoked with respect to the current runtime object.
*
* @return the <code>Runtime</code> object associated with the current
* Java application.
*/
public static Runtime getRuntime() {
return currentRuntime;
}
/** Don't let anyone else instantiate this class */
private Runtime() {}
......
}
要运行JVM中外的程序，Runtime类提供了如下方法，详细使用方法可参见源码注释
public Process exec(String command) throws IOException
public Process exec(String cmdarray[]) throws IOException
public Process exec(String command, String[] envp) throws IOException
public Process exec(String command, String[] envp, File dir) throws IOException
public Process exec(String[] cmdarray, String[] envp) throws IOException
public Process exec(String[] cmdarray, String[] envp, File dir) throws IOException
通过这种方式运行外部程序，有几个陷阱需要注意，本文尝试总结常见的几个陷阱，并给出相应的解决方法。同时封装一种比较完善的工具类，用来运行外部应用，并提供超时功能。
Runtime.exec()常见的几种陷阱以及避免方法
陷阱1：IllegalThreadStateException
通过exec执行java命令为例子，最简单的方式如下。执行exec后，通过Process获取外部进程的返回值并输出。
import java.io.IOException;
/**
* Created by yangjinfeng02 on 2016/4/27.
*/
public class Main {
public static void main(String[] args) {
Runtime runtime = Runtime.getRuntime();
try {
Process process = runtime.exec("java");
int exitVal = process.exitValue();
System.out.println("process exit value is " + exitVal);
} catch (IOException e) {
e.printStackTrace();
}
}
}
很遗憾的是，我们发现输出结果如下，抛出了IllegalThreadStateException异常
Exception in thread "main" java.lang.IllegalThreadStateException: process has not exited
at java.lang.ProcessImpl.exitValue(ProcessImpl.java:443)
at com.baidu.ubqa.agent.runner.Main.main(Main.java:18)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:497)
at com.intellij.rt.execution.application.AppMain.main(AppMain.java:144)
为什么会抛出IllegalThreadStateException异常？
这是因为外部线程还没有结束，这个时候去获取退出码，exitValue()方法抛出了异常。看到这里读者可能会问，为什么这个方法不能阻塞到外部进程结束后再返回呢？确实如此，Process有一个waitFor()方法，就是这么做的，返回的也是退出码。因此，我们可以用waitFor()方法替换exitValue()方法。
陷阱2：Runtime.exec()可能hang住，甚至死锁
首先看下Process类的文档说明
* <p>By default, the created subprocess does not have its own terminal
* or console. All its standard I/O (i.e. stdin, stdout, stderr)
* operations will be redirected to the parent process, where they can
* be accessed via the streams obtained using the methods
* {@link #getOutputStream()},
* {@link #getInputStream()}, and
* {@link #getErrorStream()}.
* The parent process uses these streams to feed input to and get output
* from the subprocess. 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, or even deadlock.
从这里可以看出，Runtime.exec()创建的子进程公用父进程的流，不同平台上，父进程的stream buffer可能被打满导致子进程阻塞，从而永远无法返回。
针对这种情况，我们只需要将子进程的stream重定向出来即可。
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
/**
* Created by yangjinfeng02 on 2016/4/27.
*/
public class Main {
public static void main(String[] args) {
Runtime runtime = Runtime.getRuntime();
try {
Process process = runtime.exec("java");
BufferedReader stdoutReader = new BufferedReader(new InputStreamReader(process.getInputStream()));
BufferedReader stderrReader = new BufferedReader(new InputStreamReader(process.getErrorStream()));
String line;
System.out.println("OUTPUT");
while ((line = stdoutReader.readLine()) != null) {
System.out.println(line);
}
System.out.println("ERROR");
while ((line = stderrReader.readLine()) != null) {
System.out.println(line);
}
int exitVal = process.waitFor();
System.out.println("process exit value is " + exitVal);
} catch (IOException e) {
e.printStackTrace();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
陷阱3：不同平台上，命令的兼容性
如果要在windows平台上运行dir命令，如果直接指定命令参数为dir，会提示命令找不到。而且不同版本windows系统上，运行改命令的方式也不一样。对这宗情况，需要根据系统版本进行适当区分。
String osName = System.getProperty("os.name" );
String[] cmd = new String[3];
if(osName.equals("Windows NT")) {
cmd[0] = "cmd.exe" ;
cmd[1] = "/C" ;
cmd[2] = args[0];
} else if(osName.equals("Windows 95")) {
cmd[0] = "command.com" ;
cmd[1] = "/C" ;
cmd[2] = args[0];
}
Runtime rt = Runtime.getRuntime();
Process proc = rt.exec(cmd);
陷阱4：错把Runtime.exec()的command参数当做命令行
本质上来讲，Runtime.exec()的command参数只是一个可运行的命令或者脚本，并不等效于Shell解器或者Cmd.exe,如果你想进行输入输出重定向，pipeline等操作，则必须通过程序来实现。不能直接在command参数中做。例如，下面的例子
Process process = runtime.exec("java -version > a.txt");
这样并不会产出a.txt文件。要达到这种目的，需通过编程手段实现，如下
import java.io.BufferedReader;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.OutputStream;
import java.io.PrintWriter;
/**
* Created by yangjinfeng02 on 2016/4/27.
*/
class StreamGobbler extends Thread {
InputStream is;
String type;
OutputStream os;
StreamGobbler(InputStream is, String type) {
this(is, type, null);
}
StreamGobbler(InputStream is, String type, OutputStream redirect) {
this.is = is;
this.type = type;
this.os = redirect;
}
public void run() {
try {
PrintWriter pw = null;
if (os != null)
pw = new PrintWriter(os);
InputStreamReader isr = new InputStreamReader(is);
BufferedReader br = new BufferedReader(isr);
String line;
while ((line = br.readLine()) != null) {
if (pw != null)
pw.println(line);
System.out.println(type + ">" + line);
}
if (pw != null)
pw.flush();
} catch (IOException ioe) {
ioe.printStackTrace();
}
}
}
public class Main {
public static void main(String args[]) {
try {
FileOutputStream fos = new FileOutputStream("logs/a.log");
Runtime rt = Runtime.getRuntime();
Process proc = rt.exec("cmd.exe /C dir");
// 重定向输出流和错误流
StreamGobbler errorGobbler = new StreamGobbler(proc.getErrorStream(), "ERROR");
StreamGobbler outputGobbler = new StreamGobbler(proc.getInputStream(), "OUTPUT", fos);
errorGobbler.start();
outputGobbler.start();
int exitVal = proc.waitFor();
System.out.println("ExitValue: " + exitVal);
fos.flush();
fos.close();
} catch (Throwable t) {
t.printStackTrace();
}
}
}
一个比较完善的工具类
下面提供一种比较完善的实现，提供了超时功能。
封装返回结果
/**
* ExecuteResult.java
*/
import lombok.Data;
import lombok.ToString;
@Data
@ToString
public class ExecuteResult {
private int exitCode;
private String executeOut;
public ExecuteResult(int exitCode, String executeOut) {
this.exitCode = exitCode;
this.executeOut = executeOut;
}
}
对外接口
/**
* LocalCommandExecutor.java
*/
public interface LocalCommandExecutor {
ExecuteResult executeCommand(String command, long timeout);
}
StreamGobbler类，用来完成stream的管理
/**
* StreamGobbler.java
*/
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
public class StreamGobbler extends Thread {
private static Logger logger = LoggerFactory.getLogger(StreamGobbler.class);
private InputStream inputStream;
private String streamType;
private StringBuilder buf;
private volatile boolean isStopped = false;
/**
* @param inputStream the InputStream to be consumed
* @param streamType the stream type (should be OUTPUT or ERROR)
*/
public StreamGobbler(final InputStream inputStream, final String streamType) {
this.inputStream = inputStream;
this.streamType = streamType;
this.buf = new StringBuilder();
this.isStopped = false;
}
/**
* Consumes the output from the input stream and displays the lines consumed
* if configured to do so.
*/
@Override
public void run() {
try {
// 默认编码为UTF-8，这里设置编码为GBK，因为WIN7的编码为GBK
InputStreamReader inputStreamReader = new InputStreamReader(inputStream, "GBK");
BufferedReader bufferedReader = new BufferedReader(inputStreamReader);
String line = null;
while ((line = bufferedReader.readLine()) != null) {
this.buf.append(line + "\n");
}
} catch (IOException ex) {
logger.trace("Failed to successfully consume and display the input stream of type " + streamType + ".", ex);
} finally {
this.isStopped = true;
synchronized (this) {
notify();
}
}
}
public String getContent() {
if (!this.isStopped) {
synchronized (this) {
try {
wait();
} catch (InterruptedException ignore) {
ignore.printStackTrace();
}
}
}
return this.buf.toString();
}
}
实现类
通过SynchronousQueue队列保证只有一个线程在获取外部进程的退出码，由线程池提供超时功能。
/**
* LocalCommandExecutorImpl.java
*/
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import java.io.Closeable;
import java.io.IOException;
import java.io.InputStream;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Future;
import java.util.concurrent.SynchronousQueue;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;
public class LocalCommandExecutorImpl implements LocalCommandExecutor {
static final Logger logger = LoggerFactory.getLogger(LocalCommandExecutorImpl.class);
static ExecutorService pool = new ThreadPoolExecutor(0, Integer.MAX_VALUE, 3L, TimeUnit.SECONDS,
new SynchronousQueue<Runnable>());
public ExecuteResult executeCommand(String command, long timeout) {
Process process = null;
InputStream pIn = null;
InputStream pErr = null;
StreamGobbler outputGobbler = null;
StreamGobbler errorGobbler = null;
Future<Integer> executeFuture = null;
try {
logger.info(command.toString());
process = Runtime.getRuntime().exec(command);
final Process p = process;
// close process's output stream.
p.getOutputStream().close();
pIn = process.getInputStream();
outputGobbler = new StreamGobbler(pIn, "OUTPUT");
outputGobbler.start();
pErr = process.getErrorStream();
errorGobbler = new StreamGobbler(pErr, "ERROR");
errorGobbler.start();
// create a Callable for the command's Process which can be called by an Executor
Callable<Integer> call = new Callable<Integer>() {
public Integer call() throws Exception {
p.waitFor();
return p.exitValue();
}
};
// submit the command's call and get the result from a
executeFuture = pool.submit(call);
int exitCode = executeFuture.get(timeout, TimeUnit.MILLISECONDS);
return new ExecuteResult(exitCode, outputGobbler.getContent());
} catch (IOException ex) {
String errorMessage = "The command [" + command + "] execute failed.";
logger.error(errorMessage, ex);
return new ExecuteResult(-1, null);
} catch (TimeoutException ex) {
String errorMessage = "The command [" + command + "] timed out.";
logger.error(errorMessage, ex);
return new ExecuteResult(-1, null);
} catch (ExecutionException ex) {
String errorMessage = "The command [" + command + "] did not complete due to an execution error.";
logger.error(errorMessage, ex);
return new ExecuteResult(-1, null);
} catch (InterruptedException ex) {
String errorMessage = "The command [" + command + "] did not complete due to an interrupted error.";
logger.error(errorMessage, ex);
return new ExecuteResult(-1, null);
} finally {
if (executeFuture != null) {
try {
executeFuture.cancel(true);
} catch (Exception ignore) {
ignore.printStackTrace();
}
}
if (pIn != null) {
this.closeQuietly(pIn);
if (outputGobbler != null && !outputGobbler.isInterrupted()) {
outputGobbler.interrupt();
}
}
if (pErr != null) {
this.closeQuietly(pErr);
if (errorGobbler != null && !errorGobbler.isInterrupted()) {
errorGobbler.interrupt();
}
}
if (process != null) {
process.destroy();
}
}
}
private void closeQuietly(Closeable c) {
try {
if (c != null) {
c.close();
}
} catch (IOException e) {
logger.error("exception", e);
}
}
}