使用Java的输入和输出重定向调用C可执行文件

Question

I have the following Folder structure:

• Project
  • Lexer
    • mylexer (this is a C executable program)
• Parser
  • MyJavaFile.java

From the java file in parser I want to execute the mylexer program and wait for a result. I have the following code:

public static String getTokensFromFile(String path) {
    String s = null;
    StringBuilder sb = new StringBuilder(path);
    try {
        Runtime rt = Runtime.getRuntime();
        String[] command = {"mylexer", "<", path, ">", "output.txt"};
        ProcessBuilder pb = new ProcessBuilder(command);
        pb.directory(new File("../Lexer"));
        Process pr = pb.start();
        BufferedReader stdInput = new BufferedReader(new
             InputStreamReader(pr.getInputStream()));

        BufferedReader stdError = new BufferedReader(new
             InputStreamReader(pr.getErrorStream()));
        while ((s = stdError.readLine()) != null) {
            sb.append(s+"\n");
        }
    }catch(Exception e) {
        System.out.println(e);
    }
    return (sb.toString().length() > 0)? sb.toString() : "";
}

I didn't get any result, the program never ends the execution, and if I do this String[] command = {"./mylexer", "<", path, ">", "output.txt"}; It says that The file is not found. How can I achieve that?

Also I did this on my terminal

../Lexer/mylexer < /Users/jacobotapia/Documents/Compiladores/Proyecto/Lexer/sample.txt > output.txt 

But this don't work on Java.

Answer 1

Input and output redirection using < and > are performed by the shell (sh, bash, or whatever you're using). They're not available in ProcessBuilder with this syntax, unless you invoke the shell from ProcessBuilder.

However ProcessBuilder has its own support for redirecting input and output of the process that it starts using the redirectInput and redirectOutput methods. The following should work for you:

String[] command = {"mylexer"};
ProcessBuilder pb = new ProcessBuilder(command);
pb.redirectInput(new File(path));
pb.redirectOutput(new File("output.txt"));