学习Java，为什么trees.getElement(treepath)返回null？

Question

我试图用解析一个java文件。

文件很差。经过几个小时的挖掘，我仍然无法让Trees#getElement为我工作。这是我的密码：

import com.sun.source.tree.*;
import com.sun.source.util.*;

import javax.tools.JavaCompiler;
import javax.tools.JavaFileObject;
import javax.tools.StandardJavaFileManager;
import javax.tools.ToolProvider;
import java.io.IOException;
import java.nio.CharBuffer;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

class CodeAnalyzerTreeVisitor extends TreePathScanner<Object, Trees> {
    @Override
    public Object visitClass(ClassTree classTree, Trees trees) {
        System.out.println("className " + classTree.getSimpleName());
        //prints name of class
        TreePath path = getCurrentPath();

        printLocationAndSource(trees, path, classTree);
        //prints the original source code

while (path != null) { System.out.println("treepath"); System.out.println(trees.getElement(path)); path = path.getParentPath(); } //it prints several nulls here //why?

        return super.visitClass(classTree, trees);
    }

    public static void printLocationAndSource(Trees trees,
                                              TreePath path, Tree tree) {
        SourcePositions sourcePosition = trees.getSourcePositions();

        long startPosition = sourcePosition.
                getStartPosition(path.getCompilationUnit(), tree);
        long endPosition = sourcePosition.
                getEndPosition(path.getCompilationUnit(), tree);

        JavaFileObject file = path.getCompilationUnit().getSourceFile();
        CharBuffer sourceContent = null;
        try {
            sourceContent = CharBuffer.wrap(file.getCharContent(true).toString().toCharArray());
        } catch (IOException e) {
            e.printStackTrace();
        }
        CharBuffer relatedSource = null;
        if (sourceContent != null) {
            relatedSource = sourceContent.subSequence((int) startPosition, (int) endPosition);
        }

        System.out.println("start: " + startPosition + " end: " + endPosition);
//        System.out.println("source: "+relatedSource);
        System.out.println();
    }
}

public class JavaParser {
    private static final JavaCompiler javac
            = ToolProvider.getSystemJavaCompiler();
    private static final String filePath = "/home/pinyin/Source/hadoop-common/hadoop-yarn-project/hadoop-ya" +
            "rn/hadoop-yarn-server/hadoop-yarn-server-resourcemanager/src/main/java/org/apache/hadoop/ya" +
            "rn/server/resourcemanager/ResourceManager.java";

    public static void main(String[] args) throws IOException {
        StandardJavaFileManager jfm = javac.getStandardFileManager(null, null, null);
        Iterable<? extends javax.tools.JavaFileObject> javaFileObjects = jfm.getJavaFileObjects(filePath);
        String[] sourcePathParam = {
                "-sourcepath",
                "/home/pinyin/Source/hadoop-common/hadoop-yarn-project/hadoop-yarn/" +
                        "hadoop-yarn-server/hadoop-yarn-server-resourcemanager/src/main/java/"
        };
        List<String> params = new ArrayList<String>();
        params.addAll(Arrays.asList(sourcePathParam));

        JavacTask task = (JavacTask) javac.getTask(null, jfm, null, params, null, javaFileObjects);

        Iterable<? extends CompilationUnitTree> asts = task.parse();
        Trees trees = Trees.instance(task);

        for (CompilationUnitTree ast : asts) {
            new CodeAnalyzerTreeVisitor().scan(ast, trees);
        }
    }
}

添加有关params和-sourcepath的行是因为我认为编译器试图在错误的位置找到源文件。他们没有工作。

我仍在试图了解Trees、javac和相关的JSR是如何协同工作的，是否有适合初学者的推荐文档？谢谢你的帮助。

编辑：，我试图分析的java文件是：https://github.com/apache/hadoop-common/blob/trunk/hadoop-yarn-project/hadoop-yarn/hadoop-yarn-server/hadoop-yarn-server-resourcemanager/src/main/java/org/apache/hadoop/yarn/server/resourcemanager/ResourceManager.java

该文件可以在其maven项目中编译而没有错误，但在我的情况下，它的依赖项不会传递给javac。我不确定这是不是问题所在。

trees.getElement在上面代码的中间部分返回null，而其他部分似乎运行良好。

Answer 1

根据this answer，在编译完成之前，Element的信息似乎是不可用的。

所以打电话给task.analyze()解决了我的问题。尽管javac正在抱怨缺少依赖关系。

如果我错了，请纠正我，谢谢。