How to add a method source to an existing java file with JDT?

I'm writing a simple eclipse plugin, which is a code generator. User can choose an existing method, then generate a new test method in corresponding test file, with JDT.

Assume the test files is already existed, and it's content is:

public class UserTest extends TestCase {
    public void setUp(){}
    public void tearDown(){}
    public void testCtor(){}
}

Now I have generate some test code:

/** complex javadoc */
public void testSetName() {
    ....
    // complex logic
}

What I want to do is to append it to the existing UserTest. I have to code:

String sourceContent = FileUtils.readFileToString("UserTest.java", "UTF-8");
ASTParser parser = ASTParser.newParser(AST.JLS3);
parser.setSource(content.toCharArray());
CompilationUnit testUnit = (CompilationUnit) parser.createAST(null);

String newTestCode = "public void testSetName() { .... }";

// get the first type
final List<TypeDeclaration> list = new ArrayList<TypeDeclaration>();
testUnit .accept(new ASTVisitor() {
    @Override
    public boolean visit(TypeDeclaration node) {
        list.add(node);
        return false;
    }
});
TypeDeclaration type = list.get(0);

type.insertMethod(newTestCode); // ! no this method: insertMethod 

But there is no such a method insertMethod.

I know two options now:

1. Do not use jdt, just to insert the new code to the test file, before last }
2. use testUnit.getAST().newMethodDeclaration() to create a method, then update it.

But I don't like these two options, I hope there is something like insertMethod, which can let me append some text to the test compilation unit, or convert the test code to a MethodDeclaration, then append to the test compilatio开发者_如何学运维n unit.

UPDATE

I see nonty's answer, and found there are two CompilationUnit in jdt. One is org.eclipse.jdt.internal.core.CompilationUnit, another is org.eclipse.jdt.core.dom.CompilationUnit. I used the second one, and nonty used the first one.

I need to supplement my question: At first I want to create a eclipse-plugin, but later I found it hard to create a complex UI by swt, so I decided to create a web app to generate the code. I copied those jdt jars from eclipse, so I can just use org.eclipse.jdt.core.dom.CompilationUnit.

Is there a way to use org.eclipse.jdt.internal.core.CompilationUnit outside eclipse?

What is wrong with

...
String newTestCode = "public void testSetName() { .... }";
IProgressMonitor monitor = new NullProgressMonitor();

IMethod method = testUnit.getTypes[0].createMethod(newTestCode, null, false, monitor);

You can even specify where to add the new method and whether to "overwrite" any exiting method.

JavaCore has a method to create a CompilationUnit from a given IFile.

IFile file;    
JavaCore.createCompilationUnitFrom(file);