resolveBinding（）方法总是返回null

Question

我正在尝试获取变量的类型信息。这是我正在使用的代码：

import org.eclipse.jdt.core.dom.AST;
import org.eclipse.jdt.core.dom.ASTParser;
import org.eclipse.jdt.core.dom.ASTVisitor;
import org.eclipse.jdt.core.dom.CompilationUnit;
import org.eclipse.jdt.core.dom.IVariableBinding;
import org.eclipse.jdt.core.dom.VariableDeclarationFragment;
import org.eclipse.jdt.core.dom.VariableDeclarationStatement;

public class ASTTester {

    public static void main(String[] args) {

        String str = "package test; import java.util.ArrayList; public class Apple {    public static void main(String[] args) {        ArrayList<String> al = null;        int j =0;   }}";
        ASTParser parser = ASTParser.newParser(AST.JLS8);
        parser.setSource(str.toCharArray());
        parser.setKind(ASTParser.K_COMPILATION_UNIT);
        parser.setResolveBindings(true);
        CompilationUnit cu = (CompilationUnit) parser.createAST(null);


        TypeFinderVisitor v = new TypeFinderVisitor();
        cu.accept(v);       
    }
}

class TypeFinderVisitor extends ASTVisitor{

    public boolean visit(VariableDeclarationStatement node){
        System.out.println(node.getType());
        for (Iterator iter = node.fragments().iterator(); iter.hasNext();) {
            VariableDeclarationFragment fragment = (VariableDeclarationFragment) iter.next();

            System.out.println(fragment.getName());

            IVariableBinding binding = fragment.resolveBinding();

            System.out.println("binding: " +binding);
        }
        return true;

    }
}

输出结果为：

ArrayList<String>
al
binding: null
int
j
binding: null

我不明白为什么绑定为null，因为我明确地将绑定设置为true。