ProcessBuilder&Runtime exec could not find or load main class in Spring Project - spring-mvc

Question

I want to compile a java file and exec its class in another class ( ← This class is a #service of a Spring MVC project ).
The service code is:
#Service
public class MRServiceImp implements MRService {
#Override
public String submitMR(int id, String fd) {
try {
// compile the java file
String[] cmd = {"javac", "P" + id + ".java"};
ProcessBuilder pb = new ProcessBuilder(cmd);
pb.directory(new File(fd));
Process p = pb.start();
// exec the class file
String[] execmd = {"java", "P" + pz_id};
ProcessBuilder epb = new ProcessBuilder(execmd);
epb.directory(new File(fd));
p = epb.start();
// get normal output
BufferedReader pin = new BufferedReader(new InputStreamReader(p.getInputStream()));
String ptmp = pin.readLine();
while (ptmp != null) {
pout = pout == null ? ptmp + '\n' : pout + ptmp + '\n';
ptmp = pin.readLine();
}
// get error output
pin = new BufferedReader(new InputStreamReader(p.getErrorStream()));
String wout = null;
ptmp = pin.readLine();
while (ptmp != null) {
wout = wout == null ? ptmp + '\n' : wout + ptmp + '\n';
ptmp = pin.readLine();
}
// print output
System.out.println(pout);
System.out.println(wout);
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
return null; // for test
}
When this Service is invoked, I always get a Error: Could not find or load main class: P[id]
I cd theFilePath, the P[id].class file is existing.
And I can run java P[id] successfully in theFilePath.
And I try to replace ProcessBuilder with Runtime, like:
#Service
public class MRServiceImp implements MRService {
#Override
public String submitMR(int id, String fd) {
try {
// compile the java file
String[] cmd = {"javac", "P" + id + ".java"};
ProcessBuilder pb = new ProcessBuilder(cmd);
pb.directory(new File(fd));
Process p = pb.start();
// exec the class file
String execmd = "java", fd + "/P" + pz_id;
p = Runtime.getRuntime().exec(execmd);
// get normal output
BufferedReader pin = new BufferedReader(new InputStreamReader(p.getInputStream()));
String ptmp = pin.readLine();
while (ptmp != null) {
pout = pout == null ? ptmp + '\n' : pout + ptmp + '\n';
ptmp = pin.readLine();
}
// get error output
pin = new BufferedReader(new InputStreamReader(p.getErrorStream()));
String wout = null;
ptmp = pin.readLine();
while (ptmp != null) {
wout = wout == null ? ptmp + '\n' : wout + ptmp + '\n';
ptmp = pin.readLine();
}
// print output
System.out.println(pout);
System.out.println(wout);
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
return null; // for test
}
I get the same Error again T^T
IDE is sts-bundle, server is tomcat8

Answer 1

I know what is wrong here.
pb.start(); does not mean the command of pb will be executed immediately.
So if I set pb of command javac hello.java; set epb of command java hello
And I call pb.start(); epb.start(); continuously, I will get an Error: could not find or load main class: hello, because when I exec epb.start(); The former command(pb.start) may have not been executed!
I got 2 solution:
First: set a finally field and exec epb.start() in this field, like:
#Service
public class MRServiceImp implements MRService {
#Override
public String submitMR(int id, String fd) {
try {
// compile the java file
String[] cmd = {"javac", "P" + id + ".java"};
ProcessBuilder pb = new ProcessBuilder(cmd);
pb.directory(new File(fd));
Process p = pb.start();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} finally {
// exec the class file
String[] execmd = {"java", "P" + pz_id};
ProcessBuilder epb = new ProcessBuilder(execmd);
epb.directory(new File(fd));
Process p = epb.start();
}
return null; // for test
}
Second: a trick of bash
#Service
public class MRServiceImp implements MRService {
#Override
public String submitMR(int id, String fd) {
try {
// compile & exec the java file
String[] cmd = {"/bin/bash"};
ProcessBuilder pb = new ProcessBuilder(cmd);
pb.directory(new File(fd));
Process p = pb.start();
BufferedWriter pbw = new BufferedWriter(new OutputStreamWriter(p.getOutputStream()));
pbw.write("javac *.java;java P" + pz_id+";exit;");
pbw.newLine();
pbw.flush();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
return null; // for test
}
I use the second one.