Engineeringserver.com | Java compiler API, compiling made easy

Java compiler API, compiling made easy

(3 votes, average: 3.33 out of 5)

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08.21.2008 by HappyFace in Java

Before Java 1.6 was released to the public I used the Runtime class (found in java.lang) to call the java compiler (javac.exe) directly to compile a Java file from another Java application, the line of code that i used looks like this

PLAIN TEXT

JAVA:

Runtime.getRuntime().exec("C:\\WINDOWS\\system32\\cmd.exe /c start javac " + userDir + "\\test\\"+className + ".java");

But luckily for me Java 1.6 introduced a handy utility that makes it possible to compile a Java application within a Java application without making direct calls to an external application. To demonstrate that I wrote a HelloWorld Java file and a CompilerConsoleDemo class that compiles the HelloWorld java file.

PLAIN TEXT

JAVA:

public class HelloWorld {
public static void main(String[] args){
System.out.println("Hello World");
}
}

I wrote the Java code below to compile the above code like this

PLAIN TEXT

JAVA:

/*--------------------------------------------------------------------------
CompilerConsoleDemo class
*****************
By: HappyFace http://www.engineeringserver.com - http://www.javaforums.net
Contact: info [@] engineeringserver.com
Version: 20/August/2008
Last updated: 20/August/2008
*****************
Note: a demo that uses the Compiler API from Java SE 1.6
//----------------------------------------------------------------------*/
import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.FileReader;
import java.io.OutputStream;
import java.util.Scanner;
import javax.tools.JavaCompiler;
import javax.tools.ToolProvider;
public class CompilerConsoleDemo{
static String userDir = System.getProperty("user.dir");
String errOutput =userDir + "out.txt";
FileReader f;
Scanner sc;
int lines = 1;
String input = null;
public static void main(String[] args){
CompilerConsoleDemo CD = new CompilerConsoleDemo(userDir + "\\HelloWorld.java");
System.out.println("Compiling:");
CD.showCode();
System.out.println();
System.out.println("Result(s):");
CD.CompileCode();
}
public CompilerConsoleDemo(String input){
this.input = input;
}
public void showCode(){
String readCode;
lines= 1;
File f = new File(input);
try {
sc = new Scanner(f);
while(sc.hasNext()){
readCode = sc.nextLine();
System.out.println(lines + " " + readCode);
lines++;
}
} catch (FileNotFoundException e) {
e.printStackTrace();
}
}
public void CompileCode(){
lines = 1;
try {
OutputStream err = new FileOutputStream(errOutput);
JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
int result = compiler.run(null,null,err,input);
if(result == 0){
System.out.println("File successfully compiled!");
System.out.println("");
System.out.println("Output:");
HelloWorld hw = ( HelloWorld ) Class.forName( "HelloWorld" ).newInstance();
hw.main(null);
}
else{
f = new FileReader(errOutput);
sc = new Scanner(f);
String readErr;
while(sc.hasNext()){
readErr = sc.nextLine();
System.out.println(lines + " " + readErr);
lines++;
}
}
} catch (FileNotFoundException e) {
e.printStackTrace();
} catch (InstantiationException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (IllegalAccessException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (ClassNotFoundException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}

The output should look like this
Compiling:
1 public class HelloWorld {
2 public static void main(String[] args){
3 System.out.println("Hello World");
4 }
5 }

Result(s):
File successfully compiled!

Output:
Hello World

But if an error occurred this will be shown instead
Compiling:
1 public class HelloWorld {
2 public static void main(String[] args){
3 System.out.println("Hello World")
4 }
5 }

Result(s):
1 C:\Eclipseworkspace\Demo\HelloWorld.java:3: ';' expected
2 System.out.println("Hello World")
3 ^
4 1 error

It looks like a really powerful utility but I haven't looked into the JavaCompiler class that much myself so there are still things that I need to figure out about the Compiler API to make maximal use it.

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