AST in JDT

如何创建一个边一单元CompilationUnit：

ASTParser parser = ASTParser.newParser(AST.JLS2);parser.setSource("".toCharArray());CompilationUnit unit = (CompilationUnit) parser.createAST(null); unit.recordModifications();AST ast = unit.getAST(); 

注意：null代表不需要ProgressMonitor

以下代码抄自eclipse.jdt的help：
AST ast = AST.newAST(AST.JLS3); //支持JDT5.0的
CompilationUnit unit = ast.newCompilationUnit();
PackageDeclaration packageDeclaration = ast.newPackageDeclaration();
packageDeclaration.setName(ast.newSimpleName("example"));
unit.setPackage(packageDeclaration);
ImportDeclaration importDeclaration = ast.newImportDeclaration();
QualifiedName name = 
ast.newQualifiedName(
ast.newSimpleName("java"),
ast.newSimpleName("util"));
importDeclaration.setName(name);
importDeclaration.setOnDemand(true);
unit.imports().add(importDeclaration);
TypeDeclaration type = ast.newTypeDeclaration();
type.setInterface(false);
type.modifiers().add(ast.newModifier(Modifier.ModifierKeyword.PUBLIC_KEYWORD));
type.setName(ast.newSimpleName("HelloWorld"));
MethodDeclaration methodDeclaration = ast.newMethodDeclaration();
methodDeclaration.setConstructor(false);
List modifiers = methodDeclaration.modifiers();
modifiers.add(ast.newModifier(Modifier.ModifierKeyword.PUBLIC_KEYWORD));
modifiers.add(ast.newModifier(Modifier.ModifierKeyword.STATIC_KEYWORD));
methodDeclaration.setName(ast.newSimpleName("main"));
methodDeclaration.setReturnType2(ast.newPrimitiveType(PrimitiveType.VOID));
SingleVariableDeclaration variableDeclaration = ast.newSingleVariableDeclaration();
variableDeclaration.setType(ast.newArrayType(ast.newSimpleType(ast.newSimpleName("String"))));
variableDeclaration.setName(ast.newSimpleName("args"));
methodDeclaration.parameters().add(variableDeclaration);
org.eclipse.jdt.core.dom.Block block = ast.newBlock();
MethodInvocation methodInvocation = ast.newMethodInvocation();
name = 
ast.newQualifiedName(
ast.newSimpleName("System"),
ast.newSimpleName("out"));
methodInvocation.setExpression(name);
methodInvocation.setName(ast.newSimpleName("println")); 
InfixExpression infixExpression = ast.newInfixExpression();
infixExpression.setOperator(InfixExpression.Operator.PLUS);
StringLiteral literal = ast.newStringLiteral();
literal.setLiteralValue("Hello");
infixExpression.setLeftOperand(literal);
literal = ast.newStringLiteral();
literal.setLiteralValue(" world");
infixExpression.setRightOperand(literal);
methodInvocation.arguments().add(infixExpression);
ExpressionStatement expressionStatement = ast.newExpressionStatement(methodInvocation);
block.statements().add(expressionStatement);
methodDeclaration.setBody(block);
type.bodyDeclarations().add(methodDeclaration);
unit.types().add(type);