亲手操作Java抽象语法树

前言

环境准备

添加JavaParser依赖

<dependency>
    <groupId>com.github.javaparsergroupId>
    <artifactId>javaparser-coreartifactId>
    <version>3.25.4version>
dependency>

第一个AST解析程序

import com.github.javaparser.StaticJavaParser;
import com.github.javaparser.ast.CompilationUnit;

public class FirstASTDemo {
    public static void main(String[] args) {
        String code = 
            "public class HelloWorld {n" +
            "    public void sayHello() {n" + 
            "        System.out.println("Hello AST!");n" +
            "    }n" +
            "}";
        
        // 解析代码为AST
        CompilationUnit cu = StaticJavaParser.parse(code);
        
        // 查看AST结构
        System.out.println("=== AST结构 ===");
        System.out.println(cu.toString());
    }
}

运行这个程序，你会看到完整的AST结构输出。

AST遍历：Visitor模式

理解Visitor设计模式

在AST处理中，Visitor模式让我们能够"访问"每个节点而不修改AST结构。

// 类比：博物馆参观者
public interface ASTVisitor {
    void visit(ClassDeclaration node);     // 参观"类展区"
    void visit(MethodDeclaration node);    // 参观"方法展区" 
    void visit(FieldDeclaration node);     // 参观"字段展区"
    // ... 其他展区
}

实现自定义Visitor

public class MethodCounter extends VoidVisitorAdapter {
    private int methodCount = 0;
    
    @Override
    public void visit(MethodDeclaration md, Void arg) {
        methodCount++;
        System.out.println("发现方法: " + md.getName() + 
                         " (参数: " + md.getParameters().size() + ")");
        super.visit(md, arg);
    }
    
    public int getMethodCount() {
        return methodCount;
    }
}

使用这个Visitor：

public class VisitorDemo {
    public static void main(String[] args) {
        String code = 
            "public class Calculator {n" +
            "    public int add(int a, int b) { return a + b; }n" +
            "    public int multiply(int a, int b) { return a * b; }n" + 
            "}";
            
        CompilationUnit cu = StaticJavaParser.parse(code);
        MethodCounter counter = new MethodCounter();
        counter.visit(cu, null);
        
        System.out.println("总方法数: " + counter.getMethodCount());
    }
}

输出：

发现方法: add (参数: 2)
发现方法: multiply (参数: 2)  
总方法数: 2

常用Visitor场景

收集所有字段信息

public class FieldCollector extends VoidVisitorAdapter {
    private List fields = new ArrayList<>();
    
    @Override 
    public void visit(FieldDeclaration fd, Void arg) {
        fd.getVariables().forEach(variable -> {
            String fieldInfo = String.format("%s %s %s", 
                fd.getModifiers(), // 访问修饰符
                fd.getElementType(), // 字段类型
                variable.getName() // 字段名
            );
            fields.add(fieldInfo);
        });
        super.visit(fd, arg);
    }
    
    public List getFields() {
        return fields;
    }
}

查找特定方法调用

public class MethodCallFinder extends VoidVisitorAdapter {
    private String targetMethod;
    private List calls = new ArrayList<>();
    
    public MethodCallFinder(String targetMethod) {
        this.targetMethod = targetMethod;
    }
    
    @Override
    public void visit(MethodCallExpr n, Void arg) {
        if (n.getNameAsString().equals(targetMethod)) {
            calls.add("在位置: " + n.getRange().map(r -> r.begin.line).orElse(-1));
        }
        super.visit(n, arg);
    }
}

AST结构可视化：理解AST树

递归打印AST树

import com.github.javaparser.StaticJavaParser;
import com.github.javaparser.ast.CompilationUnit;
import com.github.javaparser.ast.Node;

public class ASTTreeViewer {
    public static void main(String[] args) {
        String code = 
            "public class Calculator {n" +
            "    private int value;n" +
            "    n" +
            "    public int add(int a, int b) {n" +
            "        return a + b + value;n" +
            "    }n" +
            "}";
        
        CompilationUnit cu = StaticJavaParser.parse(code);
        
        System.out.println("=== AST树状结构 ===");
        printASTTree(cu, 0);
    }
    
    /**
     * 递归打印AST树结构
     * @param node 当前节点
     * @param depth 当前深度（用于缩进）
     */
    public static void printASTTree(Node node, int depth) {
        String indent = "  ".repeat(depth); // 根据深度生成缩进
        String nodeType = node.getClass().getSimpleName();
        
        // 简化的节点内容（避免输出过长）
        String content = getNodeSummary(node);
        
        System.out.println(indent + nodeType + ": " + content);
        
        // 递归打印所有子节点
        for (Node child : node.getChildNodes()) {
            printASTTree(child, depth + 1);
        }
    }
    
    /**
     * 获取节点的简化信息
     */
    private static String getNodeSummary(Node node) {
        String fullString = node.toString().split("n")[0]; // 取第一行
        if (fullString.length() > 50) {
            return fullString.substring(0, 47) + "...";
        }
        return fullString;
    }
}

运行结果示例

=== AST树状结构 ===
CompilationUnit: public class Calculator {
  ClassOrInterfaceDeclaration: public class Calculator {
    Modifier: public 
    SimpleName: Calculator
    FieldDeclaration: private int value;
      Modifier: private 
      VariableDeclarator: value
        PrimitiveType: int
        SimpleName: value
    MethodDeclaration: public int add(int a, int b) {
      Modifier: public 
      SimpleName: add
      Parameter: int a
        PrimitiveType: int
        SimpleName: a
      Parameter: int b
        PrimitiveType: int
        SimpleName: b
      PrimitiveType: int
      BlockStmt: {
        ReturnStmt: return a + b + value;
          BinaryExpr: a + b + value
            BinaryExpr: a + b
              NameExpr: a
                SimpleName: a
              NameExpr: b
                SimpleName: b
            NameExpr: value
              SimpleName: value

AST树结构解析

树的层次关系

CompilationUnit (根节点 - 整个文件)
└── ClassOrInterfaceDeclaration (类声明)
    ├── Modifier (修饰符: public)
    ├── SimpleName (类名: Calculator)  
    ├── FieldDeclaration (字段声明)
    │   ├── Modifier (修饰符: private)
    │   └── VariableDeclarator (变量声明器)
    │       └── SimpleName (字段名: value)
    └── MethodDeclaration (方法声明)
        ├── Modifier (修饰符: public)
        ├── SimpleName (方法名: add)
        ├── Parameter (参数)
        │   ├── PrimitiveType (类型: int)
        │   └── SimpleName (参数名: a)
        ├── BlockStmt (方法体)
        │   └── ReturnStmt (返回语句)
        │       └── BinaryExpr (二元表达式: a + b + value)
        │           ├── BinaryExpr (子表达式: a + b)
        │           │   ├── NameExpr (变量a)
        │           │   └── NameExpr (变量b)
        │           └── NameExpr (变量value)

关键节点类型说明

AST修改：实现代码自动生成

为类添加字段

public class ClassModifier {
    public static void addField(CompilationUnit cu, 
                               String fieldName, 
                               String fieldType) {
        // 为 user 类添加字段
        cu.getClassByName("User").ifPresent(clazz -> {
            // 构建字段代码
            String fieldCode = "private " + fieldType + " " + fieldName + ";";
            
            // 解析为AST节点并添加
            var field = StaticJavaParser.parseBodyDeclaration(fieldCode);
            clazz.addMember(field);
            
            System.out.println("成功添加字段: " + fieldName);
        });
    }
}

自动生成Getter方法

public class GetterGenerator extends VoidVisitorAdapter {
    @Override
    public void visit(ClassOrInterfaceDeclaration cid, Void arg) {
        // 为每个字段生成getter
        cid.getFields().forEach(field -> {
            field.getVariables().forEach(variable -> {
                String fieldName = variable.getNameAsString();
                String fieldType = field.getElementType().asString();
                generateGetter(cid, fieldName, fieldType);
            });
        });
        super.visit(cid, arg);
    }
    
    private void generateGetter(ClassOrInterfaceDeclaration clazz, 
                               String fieldName, String fieldType) {
        String getterName = "get" + capitalize(fieldName);
        String getterCode = String.format(
            "public %s %s() { return this.%s; }",
            fieldType, getterName, fieldName
        );
        
        try {
            var getter = StaticJavaParser.parseBodyDeclaration(getterCode);
            clazz.addMember(getter);
            System.out.println("生成Getter: " + getterName);
        } catch (Exception e) {
            System.err.println("生成Getter失败: " + e.getMessage());
        }
    }
    
    private String capitalize(String str) {
        return str.substring(0, 1).toUpperCase() + str.substring(1);
    }
}