Return value from a Java code

问题:

There is a Java class which creates a POST request and sends it to a servlet. The main method of the class file (test) looks something like this:

public static void main(String[] args) throws IOException {
  // Code logic goes here...
  // No return Statement
}

This is called from a KornShell (ksh) script something like this:

retcode=`$CLK_JAVA_PATH -cp $CLASSPATH test ${PASSWORD} ${HOSTNAME} ${TOOLSET}`

if [ $? != "0" ];then
        echo "ERROR:  
        echo "${retcode}"
else
        echo "${SCRIPT} Success"
fi

retcode always has the value "2" independent of if the code fails or succeeds. My question is since the return type of my main method is "void" why is the code returning some value?

回答1:

The return value of a Java application is not the return value of it's main method, because a Java application doesn't necessarily end when it's main method has finished execution.

Instead the JVM ends when no more non-daemon threads are running or when System.exit() is called.

And System.exit() is also the only way to specify the return value: the argument passed to System.exit() will be used as the return value of the JVM process on most OS.

So ending your main() method with this:

System.exit(0);

will ensure two things:

• that your Java application really exits when the end of main is reached and
• that the return value of the JVM process is 0

回答2:

Java programs do not return an exit code back to the operating system by returning a value from main, as is done in C and C++. You can exit the program and specify the exit code by calling System.exit(code);, for example:

// Returns exit code 2 to the operating system
System.exit(2);

回答3:

System.exit(0);

This returns error code 0 (everything went fine). System.exit Doc

回答4:

Use

System.exit( someNumber );

this will give your app control over the return value seen by the OS.

回答5:

Your program always returns a return code after exiting. In normal programs, if you do not specify a return code, it will return zero (this includes setting the return type to void).

Java, however, likes to be special! Java won't return the return code you return at the Main method, but it'll return some return code when the JVM exits (this accounts for multithreaded programs), and will return what a System.Exit(returnCode); call specifies.

回答6:

You're not getting the exit status, that's what $? contains. You're getting standard out, whatever is written to System.out.