Is it possible to programmatically compile java source code in memory only?

Question

I have found many references explaining how to programmatically compile a Java class using the JavaCompiler class:

JavaCompiler compiler = ToolProvider.getSystemJavaCompiler(); int result = compiler.run(null, null, null, "a_file_name"); 

However, I would like to know if there is an open source library that let me compile source code generated programmatically (therefore without a src file being involved) and generate some byte code in an output stream (without generating a class file in the file system).

For example, I am looking for being able to write something like this:

InputStream input = generateSourceCode(); OutputStream output = getByteCode(input); doCoolStuffWithByteCode(output); 

Thanks for any help.

Answer 1

To start, look at the JavaCompiler API. Basically:

1. Create the Java class in a string.
2. Put the string into class that extends SimpleJavaFileObject.
3. Compile using a JavaCompiler instance.

Finally, call the methods the new class.

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Here is an example that works with JDK6+:

import java.io.IOException; import java.io.PrintWriter; import java.io.StringWriter; import java.lang.reflect.InvocationTargetException; import java.net.URI; import java.util.Arrays;  import javax.tools.Diagnostic; import javax.tools.DiagnosticCollector; import javax.tools.JavaCompiler; import javax.tools.JavaFileObject; import javax.tools.SimpleJavaFileObject; import javax.tools.ToolProvider; import javax.tools.JavaCompiler.CompilationTask; import javax.tools.JavaFileObject.Kind;  public class CompileSourceInMemory {   public static void main(String args[]) throws IOException {     JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();     DiagnosticCollector<JavaFileObject> diagnostics = new DiagnosticCollector<JavaFileObject>();      StringWriter writer = new StringWriter();     PrintWriter out = new PrintWriter(writer);     out.println("public class HelloWorld {");     out.println("  public static void main(String args[]) {");     out.println("    System.out.println(\"This is in another java file\");");         out.println("  }");     out.println("}");     out.close();     JavaFileObject file = new JavaSourceFromString("HelloWorld", writer.toString());      Iterable<? extends JavaFileObject> compilationUnits = Arrays.asList(file);     CompilationTask task = compiler.getTask(null, null, diagnostics, null, null, compilationUnits);      boolean success = task.call();     for (Diagnostic diagnostic : diagnostics.getDiagnostics()) {       System.out.println(diagnostic.getCode());       System.out.println(diagnostic.getKind());       System.out.println(diagnostic.getPosition());       System.out.println(diagnostic.getStartPosition());       System.out.println(diagnostic.getEndPosition());       System.out.println(diagnostic.getSource());       System.out.println(diagnostic.getMessage(null));      }     System.out.println("Success: " + success);      if (success) {       try {         Class.forName("HelloWorld").getDeclaredMethod("main", new Class[] { String[].class })             .invoke(null, new Object[] { null });       } catch (ClassNotFoundException e) {         System.err.println("Class not found: " + e);       } catch (NoSuchMethodException e) {         System.err.println("No such method: " + e);       } catch (IllegalAccessException e) {         System.err.println("Illegal access: " + e);       } catch (InvocationTargetException e) {         System.err.println("Invocation target: " + e);       }     }   } }  class JavaSourceFromString extends SimpleJavaFileObject {   final String code;    JavaSourceFromString(String name, String code) {     super(URI.create("string:///" + name.replace('.','/') + Kind.SOURCE.extension),Kind.SOURCE);     this.code = code;   }    @Override   public CharSequence getCharContent(boolean ignoreEncodingErrors) {     return code;   } }