Java 调用 shell 脚本详解
这一年的项目中，有大量的场景需要Java 进程调用 Linux的bash shell 脚本实现相关功能。
从之前的项目中拷贝的相关模块和网上的例子来看，有个别的“陷阱”造成调用shell 脚本在某些特殊的场景下，有一些奇奇怪怪的bug。
大家且听我一一道来。
先看看网上搜索到的例子：
package someTest;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
public class ShellTest {
public static void main(String[] args) {
InputStreamReader stdISR = null;
InputStreamReader errISR = null;
Process process = null;
String command = "/home/Lance/workspace/someTest/testbash.sh";
try {
process = Runtime.getRuntime().exec(command);
int exitValue = process.waitFor();
String line = null;
stdISR = new InputStreamReader(process.getInputStream());
BufferedReader stdBR = new BufferedReader(stdISR);
while ((line = stdBR.readLine()) != null) {
System.out.println("STD line:" + line);
}
errISR = new InputStreamReader(process.getErrorStream());
BufferedReader errBR = new BufferedReader(errISR);
while ((line = errBR.readLine()) != null) {
System.out.println("ERR line:" + line);
}
} catch (IOException | InterruptedException e) {
e.printStackTrace();
} finally {
try {
if (stdISR != null) {
stdISR.close();
}
if (errISR != null) {
errISR.close();
}
if (process != null) {
process.destroy();
}
} catch (IOException e) {
System.out.println("正式执行命令：" + command + "有IO异常");
}
}
}
}
testbash.sh
#!/bin/bash
echo `pwd`输出结果为：
STD line:/home/Lance/workspace/someTestJava在执行Runtime.getRuntime().exec(command)之后，Linux会创建一个进程，该进程与JVM进程建立三个管道连接，标准输入流、标准输出流、标准错误流。
上述代码，依次读取标准输出流和标准错误流，在shell给出“退出信号”后，做了相应的清理工作。
对于一般场景来说，这段代码可以凑合用了。但是，在实际场景中，会有以下几个“陷阱”。
一. 当标准输出流或标准错误流非常庞大的时候，会出现调用waitFor方法卡死的bug。
真实的环境中，当标准输出在10000行左右的时候，就会出现卡死的情况。
原因分析：假设linux进程不断向标准输出流和标准错误流写数据，而JVM却不读取，数据会暂存在linux缓存区，当缓存区存满之后导致该进程无法继续写数据，会僵死，导致java进程会卡死在waitFor()处，永远无法结束。
解决方式：由于标准输出和错误输出都会向Linux缓存区写数据，而脚本如何输出这两种流是Java端不能确定的。为了不让shell脚本的子进程卡死，这两种输出需要分别读取，而且不能互相影响。所以必须新开两个线程来进行读取。
我开始的实现如下：
package someTest;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.util.LinkedList;
import java.util.List;
public class CommandStreamGobbler extends Thread {
private InputStream is;
private String command;
private String prefix = "";
private boolean readFinish = false;
private boolean ready = false;
private List infoList = new LinkedList();
CommandStreamGobbler(InputStream is, String command, String prefix) {
this.is = is;
this.command = command;
this.prefix = prefix;
}
public void run() {
InputStreamReader isr = null;
try {
isr = new InputStreamReader(is);
BufferedReader br = new BufferedReader(isr);
String line = null;
ready = true;
while ((line = br.readLine()) != null) {
infoList.add(line);
System.out.println(prefix + " line: " + line);
}
} catch (IOException ioe) {
System.out.println("正式执行命令：" + command + "有IO异常");
} finally {
try {
if (isr != null) {
isr.close();
}
} catch (IOException ioe) {
System.out.println("正式执行命令：" + command + "有IO异常");
}
readFinish = true;
}
}
public InputStream getIs() {
return is;
}
public String getCommand() {
return command;
}
public boolean isReadFinish() {
return readFinish;
}
public boolean isReady() {
return ready;
}
public List getInfoList() {
return infoList;
}
}
package someTest;
import java.io.IOException;
import java.io.InputStreamReader;
public class ShellTest {
public static void main(String[] args) {
InputStreamReader stdISR = null;
InputStreamReader errISR = null;
Process process = null;
String command = "/home/Lance/workspace/someTest/testbash.sh";
try {
process = Runtime.getRuntime().exec(command);
CommandStreamGobbler errorGobbler = new CommandStreamGobbler(process.getErrorStream(), command, "ERR");
CommandStreamGobbler outputGobbler = new CommandStreamGobbler(process.getInputStream(), command, "STD");
errorGobbler.start();
// 必须先等待错误输出ready再建立标准输出
while (!errorGobbler.isReady()) {
Thread.sleep(10);
}
outputGobbler.start();
while (!outputGobbler.isReady()) {
Thread.sleep(10);
}
int exitValue = process.waitFor();
} catch (IOException | InterruptedException e) {
e.printStackTrace();
} finally {
try {
if (stdISR != null) {
stdISR.close();
}
if (errISR != null) {
errISR.close();
}
if (process != null) {
process.destroy();
}
} catch (IOException e) {
System.out.println("正式执行命令：" + command + "有IO异常");
}
}
}
}
到此为止，解决了Java卡死shell脚本的情况。再说说，第二种可能。
二. 由于shell脚本的编写问题，当其自身出现僵死的情况，上述代码出现Java代码被僵死的Shell脚本阻塞住的情况。
原因分析：由于shell脚本也是人写的，难免会出现失误。在Java调用shell脚本时，无论是Debug场景还是生产环境，都发生过shell脚本意外僵死反过来卡死Java相关线程的情况。典型的表现为：shell脚本长时间运行，标准输出和错误输出没有任何输出（包括结束符），操作系统显示shell脚本在正常运行或僵死，没有退出信号。
解决方式：上述代码中，至少有三处会导致线程阻塞，包括标准输出和错误输出这线程的BufferedReader的readline方法，以及Process的waitFor方法。解决这个问题的核心有两个，1.避免任何Java线程被阻塞住，因为一旦被IO阻塞住，线程将处于内核态，主线程没有任何办法强制结束相关子线程。2.添加一个简单的超时机制，超时后回收相应的线程资源，并结束调用过程。
演示代码中，我改写了testshell.sh，写一个没有任何输出的死循环模拟shell卡死的情况。
#!/bin/bash
while true;do
a=1
sleep 0.1
donepackage someTest;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.util.LinkedList;
import java.util.List;
public class CommandStreamGobbler extends Thread {
private InputStream is;
private String command;
private String prefix = "";
private boolean readFinish = false;
private boolean ready = false;
// 命令执行结果,0:执行中 1:超时 2:执行完成
private int commandResult = 0;
private List infoList = new LinkedList();
CommandStreamGobbler(InputStream is, String command, String prefix) {
this.is = is;
this.command = command;
this.prefix = prefix;
}
public void run() {
InputStreamReader isr = null;
BufferedReader br = null;
try {
isr = new InputStreamReader(is);
br = new BufferedReader(isr);
String line = null;
ready = true;
while (commandResult != 1) {
if (br.ready() || commandResult == 2) {
if ((line = br.readLine()) != null) {
infoList.add(line);
} else {
break;
}
} else {
Thread.sleep(100);
}
}
} catch (IOException | InterruptedException ioe) {
System.out.println("正式执行命令：" + command + "有IO异常");
} finally {
try {
if (br != null) {
br.close();
}
if (isr != null) {
isr.close();
}
} catch (IOException ioe) {
System.out.println("正式执行命令：" + command + "有IO异常");
}
readFinish = true;
}
}
public InputStream getIs() {
return is;
}
public String getCommand() {
return command;
}
public boolean isReadFinish() {
return readFinish;
}
public boolean isReady() {
return ready;
}
public List getInfoList() {
return infoList;
}
public void setTimeout(int timeout) {
this.commandResult = timeout;
}
}
package someTest;
public class CommandWaitForThread extends Thread {
private Process process;
private boolean finish = false;
private int exitValue = -1;
public CommandWaitForThread(Process process) {
this.process = process;
}
public void run() {
try {
this.exitValue = process.waitFor();
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
finish = true;
}
}
public boolean isFinish() {
return finish;
}
public void setFinish(boolean finish) {
this.finish = finish;
}
public int getExitValue() {
return exitValue;
}
}
package someTest;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.Date;
public class ShellTest {
public static void main(String[] args) {
InputStreamReader stdISR = null;
InputStreamReader errISR = null;
Process process = null;
String command = "/home/Lance/workspace/someTest/testbash.sh";
long timeout = 10 * 1000;
try {
process = Runtime.getRuntime().exec(command);
CommandStreamGobbler errorGobbler = new CommandStreamGobbler(process.getErrorStream(), command, "ERR");
CommandStreamGobbler outputGobbler = new CommandStreamGobbler(process.getInputStream(), command, "STD");
errorGobbler.start();
// 必须先等待错误输出ready再建立标准输出
while (!errorGobbler.isReady()) {
Thread.sleep(10);
}
outputGobbler.start();
while (!outputGobbler.isReady()) {
Thread.sleep(10);
}
CommandWaitForThread commandThread = new CommandWaitForThread(process);
commandThread.start();
long commandTime = new Date().getTime();
long nowTime = new Date().getTime();
boolean timeoutFlag = false;
while (!commandIsFinish(commandThread, errorGobbler, outputGobbler)) {
if (nowTime - commandTime > timeout) {
timeoutFlag = true;
break;
} else {
Thread.sleep(100);
nowTime = new Date().getTime();
}
}
if (timeoutFlag) {
// 命令超时
errorGobbler.setTimeout(1);
outputGobbler.setTimeout(1);
System.out.println("正式执行命令：" + command + "超时");
}else {
// 命令执行完成
errorGobbler.setTimeout(2);
outputGobbler.setTimeout(2);
}
while (true) {
if (errorGobbler.isReadFinish() && outputGobbler.isReadFinish()) {
break;
}
Thread.sleep(10);
}
} catch (IOException | InterruptedException e) {
e.printStackTrace();
} finally {
if (process != null) {
process.destroy();
}
}
}
private boolean commandIsFinish(CommandWaitForThread commandThread, CommandStreamGobbler errorGobbler, CommandStreamGobbler outputGobbler) {
if (commandThread != null) {
return commandThread.isFinish();
} else {
return (errorGobbler.isReadFinish() && outputGobbler.isReadFinish());
}
}
}
1. 在CommandStreamGobbler里，bufferedReader在readLine()之前，先用ready()看一下当前缓冲区的情况，请特别注意ready()描述，这个方法是非阻塞的。
boolean java.io.BufferedReader.ready() throws IOException
Tells whether this stream is ready to be read. A buffered character stream is ready if the buffer is not empty, or if the underlying character stream is ready.
Returns:
True if the next read() is guaranteed not to block for input, false otherwise. Note that returning false does not guarantee that the next read will block.以上的两点改进，保证了Java在调用shell脚本过程互不被对方卡死的机制。
三.在执行shell脚本过程中，可能会添加参数，通常在终端中，我们使用“ ”（空格）把参数隔开。
为了区分空格是作为参数分隔符，还是参数的一部分。调用exec方法有特别的注意事项。
String command = "/home/Lance/workspace/someTest/testbash.sh 'hello world'";
process = Runtime.getRuntime().exec(command);等价于
List commandList = new LinkedList();
commandList.add("/home/Lance/workspace/someTest/testbash.sh");
commandList.add("hello world");
String[] commands = new String[commandList.size()];
for (int i = 0; i < commandList.size(); i++) {
commands[i] = commandList.get(i);
}
process = Runtime.getRuntime().exec(commands); 好了，今天介绍到这里。