[ Java 常見問題 ] 執行外部程式等不到 Return code

轉載自 這裡 
前言 : 
程式因為某種需要要去呼叫外部程式, 要等待外部程式執行完後再繼續進行, 如下所示 : 

- 部分範例代碼 :

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1. Runtime rt = Runtime.getRuntime();  
2. Process proc = rt.exec(exec);  
3. System.out.println("Retrun Code:" + proc.waitFor());  

但是執行外部程式時,程式的控制權就不再我手上,會一直waitFor(). 導致我的程式無法再繼續往下走. 

解法 : 
之前整好我也遇到這個問題. Runtime().exec()的 waitFor需要把inputStream和errorStream清空才會把控制權給你,否則就一直在wait中. 底下是摘自《More Java pitfalls 中文版》上的例子 : 

- BadExecJava2.java 代碼 :

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1. package test;  
2.   
3. import java.util.*;  
4. import java.io.*;  
5.   
6. public class BadExecJava2 {  
7.     public static void main(String args[]) {  
8.         try {  
9.             Runtime rt = Runtime.getRuntime();  
10.             Process proc = rt.exec("javac");  
11.             int exitVal = proc.waitFor();  
12.             System.out.println("Process exitValue: " + exitVal);  
13.         } catch (Throwable t) {  
14.             t.printStackTrace();  
15.         }  
16.     }  
17. }  

我們知道javac 命令，當不帶參數運行javac 程序時，它將輸出幫助說明，為什麼上面程序不產生任何輸出並掛起，永不完成呢？java文檔上說,由於有些本地平台為標准輸入和輸出流所提供的緩沖區大小有限,如果不能及時寫入子進程的輸入流或者讀取子進程的輸出流,可能導致子進程阻塞,甚至陷入死鎖。所以，上面的程序應改寫為 : 

- MediocreExecJavac.java 代碼 :

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1. package test;  
2.   
3. import java.io.*;  
4.   
5. public class MediocreExecJavac {  
6.     public static void main(String args[]) {  
7.         try {  
8.             Runtime rt = Runtime.getRuntime();  
9.             Process proc = rt.exec("javac");  
10.             InputStream stderr = proc.getErrorStream();  
11.             InputStreamReader isr = new InputStreamReader(stderr);  
12.             BufferedReader br = new BufferedReader(isr);  
13.             String line = null;  
14.             System.out.println("");  
15.             while ((line = br.readLine()) != null)  
16.                 System.out.println(line);  
17.             System.out.println("");  
18.             int exitVal = proc.waitFor();  
19.             System.out.println("Process exitValue: " + exitVal);  
20.         } catch (Throwable t) {  
21.             t.printStackTrace();  
22.         }  
23.     }  
24. }  

下面是正確的輸出 : 

- 執行輸出 :

Usage: javac
where possible options include:
-g Generate all debugging info
-g:none Generate no debugging info
...(中間省略)...
-Akey[=value] Options to pass to annotation processors
-X Print a synopsis of nonstandard options
-J Pass directly to the runtime system

Process exitValue: 2

下面是一個更一般的程序，它用兩個線程同步清空標准錯誤流和標准輸出流，並能根據你所使用的windows操作系統選擇windows命令解釋器command.com或cmd.exe,然後執行你提供的命令 : 

- StreamGobbler.java 代碼 :

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1. package test;  
2.   
3. import java.util.*;  
4. import java.io.*;  
5.   
6. class StreamGobbler extends Thread {  
7.     InputStream is;  
8.     String type; // 輸出流的類型ERROR或OUTPUT  
9.   
10.     StreamGobbler(InputStream is, String type) {  
11.         this.is = is;  
12.         this.type = type;  
13.     }  
14.   
15.     public void run() {  
16.         try {  
17.             InputStreamReader isr = new InputStreamReader(is);  
18.             BufferedReader br = new BufferedReader(isr);  
19.             String line = null;  
20.             while ((line = br.readLine()) != null) {  
21.                 System.out.println(type + ">" + line);  
22.                 System.out.flush();  
23.             }  
24.         } catch (IOException ioe) {  
25.             ioe.printStackTrace();  
26.         }  
27.     }  
28. }  

- GoodWindowsExec.java 代碼 :

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1. package test;  
2.   
3. public class GoodWindowsExec {  
4.     public static void main(String args[]) {  
5.         if (args.length < 1) {  
6.             System.out.println("USAGE: java GoodWindowsExec ");  
7.             System.exit(1);  
8.         }  
9.   
10.         try {  
11.             String osName = System.getProperty("os.name");  
12.             System.out.println("osName: " + osName);  
13.             String[] cmd = new String[3];  
14.   
15.             if (osName.equals("Windows XP") || osName.equals("Windows 2000")) {  
16.                 cmd[0] = "cmd.exe";  
17.                 cmd[1] = "/C";  
18.                 cmd[2] = args[0];  
19.             } else if (osName.equals("Windows 98")) {  
20.                 cmd[0] = "command.com";  
21.                 cmd[1] = "/C";  
22.                 cmd[2] = args[0];  
23.             }  
24.   
25.             Runtime rt = Runtime.getRuntime();  
26.             System.out.println("Execing " + cmd[0] + " " + cmd[1] + " "  
27.                     + cmd[2]);  
28.             Process proc = rt.exec(cmd);  
29.             // any error message?  
30.             StreamGobbler errorGobbler = new StreamGobbler(proc  
31.                     .getErrorStream(), "ERROR");  
32.             // any output?  
33.             StreamGobbler outputGobbler = new StreamGobbler(proc  
34.                     .getInputStream(), "OUTPUT");  
35.   
36.             // kick them off  
37.             errorGobbler.start();  
38.             outputGobbler.start();  
39.   
40.             // any error???  
41.             int exitVal = proc.waitFor();  
42.             System.out.println("ExitValue: " + exitVal);  
43.   
44.         } catch (Throwable t) {  
45.             t.printStackTrace();  
46.         }  
47.     }  
48. }