Executing one Java program from another using Runtime.exec(...)

Question

The tester program that I wanted execute takes in a single argument -- a filename -- and makes a copy of the file with the line "This is a modified version." at the top of the new file. When I tested this program alone, it works and produces a new file.

Then I wrote the program to call the file:

public static void main(String[] args) {
    try {
        Process p = Runtime.getRuntime.exec("java Tester.java inputfilename.txt");
        p.waitFor();
        System.out.println("Done");
    } catch(Exception e) {
        System.out.println("Error");
        System.exit(0);
    }
}

The program above printed out "Done" but it never made a modified version of the file I passed in. I then put some println()'s in the other program. When I run that program alone, it printed out those statements, but when I tried to call it from the program above, it did not. How do I fix this?

Jari told you why it did not work in general. But concerning the output: You explicitly have to *read* the output that is generated by the program that you start there. Also look at http://stackoverflow.com/a/21904185 — Marco13, Mar 23 '14 at 19:03

score 3 · Accepted Answer · answered Mar 23 '14 at 19:02

3

You have to compile .java file first and launch it later:

Compile (class containing the main method):

javac Tester.java

Launch:

java Tester inputfilename.txt

answered Mar 23 '14 at 19:02

sybear

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I added (before the line:Process p = Runtime.getRuntime.exec("java Tester.java inputfilename.txt");) the line: Runtime.getRuntime().exec("javac Tester.java"); and then ran the program. It still didn't work. Is my syntax wrong? – user3025403 Mar 23 '14 at 19:17
Open `cmd`, run this stuff manually and tell me what errors you get. – sybear Mar 23 '14 at 19:34
The message I got: Exception in thread "main" java.lang.NoClassDefFoundError: Tester/java Caused by: java.lang.ClassNotFoundException: Tester.java at java.net.URLClassLoader$1.run(URLClassLoader.java:202) at java.security.AccessController.doPrivileged(Native Method) at java.net.URLClassLoader.findClass(URLClassLoader.java:190) at java.lang.ClassLoader.loadClass(ClassLoader.java:306) at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301) at java.lang.ClassLoader.loadClass(ClassLoader.java:247) – user3025403 Mar 23 '14 at 19:43
Well, that a different question, check this http://stackoverflow.com/questions/6334148/exception-in-thread-main-java-lang-noclassdeffounderror or just google for it. – sybear Mar 23 '14 at 19:54
`.NoClassDefFoundError: Tester/java` indicates you are **still** adding the `.java` extension when using the `java` command! Jari advised you around an hour before that, to *remove* it! – Andrew Thompson Mar 24 '14 at 06:39
Thanks Andrew. I figured that out yesterday and ran the program without any issues. – user3025403 Mar 26 '14 at 21:57

score 1 · Answer 2 · answered Mar 24 '14 at 06:36

"java Tester.java inputfilename.txt"

Should be:

"java Tester inputfilename.txt"

But do yourself a favor and read (and implement) all the recommendations of When Runtime.exec() won't.

That might solve other problems. If not, it should provide more information as to the reason for failure.

Then ignore that it refers to exec and build the Process using a ProcessBuilder. Also break a String arg into String[] args to account for arguments which themselves contain spaces.

Executing one Java program from another using Runtime.exec(...)

2 Answers2