JAVA 8 反射获取参数名

摘要：前言在之前编译是不会把构造器和方法的参数名编译进中，如果需要获取参数名，可以在方法上加上注解，反射获取注解的值从而获取参数名，比如的和。带在中添加命令行在后面加运行结果构造器方法一方法二方法三方法四这样就把参数名给打印出来了，为。

在JDK8之前javac编译是不会把构造器和方法的参数名编译进class中，如果需要获取参数名，可以在方法上加上注解，反射获取注解的值从而获取参数名，比如Jackson的@JsonCreator和@JsonProperty 。而JDK8新增了这一个功能，可以直接调用java.lang.reflect.Parameter.getName()获取到，前提是javac需要添加-parameters这个参数。通常来说不建议这样做，因为这会增大.class和在JVM中会占用更多的内存。

直接上代码。

用来打印类信息

import java.lang.reflect.Constructor;
import java.lang.reflect.Method;
import java.lang.reflect.Parameter;

import static java.lang.System.out;

public class MethodParameterSpy {

    private static final String  fmt = "%24s: %s%n";


    public static void printClassConstructors(Class c) {
        Constructor[] allConstructors = c.getConstructors();
        out.format(fmt, "Number of constructors", allConstructors.length);
        for (Constructor currentConstructor : allConstructors) {
            printConstructor(currentConstructor);
        }
        Constructor[] allDeclConst = c.getDeclaredConstructors();
        out.format(fmt, "Number of declared constructors",
                allDeclConst.length);
        for (Constructor currentDeclConst : allDeclConst) {
            printConstructor(currentDeclConst);
        }
    }

    public static void printClassMethods(Class c) {
        Method[] allMethods = c.getDeclaredMethods();
        out.format(fmt, "Number of methods", allMethods.length);
        for (Method m : allMethods) {
            printMethod(m);
        }
    }

    public static void printConstructor(Constructor c) {
        out.format("%s%n", c.toGenericString());
        Parameter[] params = c.getParameters();
        out.format(fmt, "Number of parameters", params.length);
        for (int i = 0; i < params.length; i++) {
            printParameter(params[i]);
        }
    }

    public static void printMethod(Method m) {
        out.format("%s%n", m.toGenericString());
        out.format(fmt, "Return type", m.getReturnType());
        out.format(fmt, "Generic return type", m.getGenericReturnType());

        Parameter[] params = m.getParameters();
        for (int i = 0; i < params.length; i++) {
            printParameter(params[i]);
        }
    }

    public static void printParameter(Parameter p) {
        out.format(fmt, "Parameter class", p.getType());
        out.format(fmt, "Parameter name", p.getName());
        out.format(fmt, "Modifiers", p.getModifiers());
        out.format(fmt, "Is implicit?", p.isImplicit());
        out.format(fmt, "Is name present?", p.isNamePresent());
        out.format(fmt, "Is synthetic?", p.isSynthetic());
    }

    public static void main(String... args) {

        printClassConstructors(ExampleMethods.class);
        printClassMethods(ExampleMethods.class);
    }
}

包含各种类型方法的类

import java.util.*;

public class ExampleMethods {

    public boolean simpleMethod(String stringParam, int intParam) {
        System.out.println("String: " + stringParam + ", integer: " + intParam);
        return true;
    }

    public int varArgsMethod(String... manyStrings) {
        return manyStrings.length;
    }

    public boolean methodWithList(List listParam) {
        return listParam.isEmpty();
    }

    public  void genericMethod(T[] a, Collection c) {
        System.out.println("Length of array: " + a.length);
        System.out.println("Size of collection: " + c.size());
    }

}

不带-parameters运行结果：

  Number of constructors: 1
public ExampleMethods()
    Number of parameters: 0
Number of declared constructors: 1

# 构造器
public ExampleMethods()
    Number of parameters: 0
       Number of methods: 4
       
# 方法一
public boolean ExampleMethods.simpleMethod(java.lang.String,int)
             Return type: boolean
     Generic return type: boolean
         Parameter class: class java.lang.String
          Parameter name: arg0
               Modifiers: 0
            Is implicit?: false
        Is name present?: false
           Is synthetic?: false
         Parameter class: int
          Parameter name: arg1
               Modifiers: 0
            Is implicit?: false
        Is name present?: false
           Is synthetic?: false
           
# 方法二
public boolean ExampleMethods.methodWithList(java.util.List)
             Return type: boolean
     Generic return type: boolean
         Parameter class: interface java.util.List
          Parameter name: arg0
               Modifiers: 0
            Is implicit?: false
        Is name present?: false
           Is synthetic?: false

# 方法三
public  void ExampleMethods.genericMethod(T[],java.util.Collection)
             Return type: void
     Generic return type: void
         Parameter class: class [Ljava.lang.Object;
          Parameter name: arg0
               Modifiers: 0
            Is implicit?: false
        Is name present?: false
           Is synthetic?: false
         Parameter class: interface java.util.Collection
          Parameter name: arg1
               Modifiers: 0
            Is implicit?: false
        Is name present?: false
           Is synthetic?: false

# 方法四
public int ExampleMethods.varArgsMethod(java.lang.String...)
             Return type: int
     Generic return type: int
         Parameter class: class [Ljava.lang.String;
          Parameter name: arg0
               Modifiers: 0
            Is implicit?: false
        Is name present?: false
           Is synthetic?: false

可以看出Parameter name全都是arg0~argN，因为参数名在编译期已经丢失了。Is name present为false。

maven在pom.xml中添加

    
        
            org.apache.maven.plugins
            maven-compiler-plugin
            
                8
                8
                -parameters
            
        
    

命令行在javac 后面加 -parameters

运行结果

  Number of constructors: 1
public ExampleMethods()
    Number of parameters: 0
Number of declared constructors: 1

# 构造器
public ExampleMethods()
    Number of parameters: 0
       Number of methods: 4
           
# 方法一
public boolean ExampleMethods.methodWithList(java.util.List)
             Return type: boolean
     Generic return type: boolean
         Parameter class: interface java.util.List
          Parameter name: listParam
               Modifiers: 0
            Is implicit?: false
        Is name present?: true
           Is synthetic?: false

# 方法二
public int ExampleMethods.varArgsMethod(java.lang.String...)
             Return type: int
     Generic return type: int
         Parameter class: class [Ljava.lang.String;
          Parameter name: manyStrings
               Modifiers: 0
            Is implicit?: false
        Is name present?: true
           Is synthetic?: false

# 方法三
public  void ExampleMethods.genericMethod(T[],java.util.Collection)
             Return type: void
     Generic return type: void
         Parameter class: class [Ljava.lang.Object;
          Parameter name: a
               Modifiers: 0
            Is implicit?: false
        Is name present?: true
           Is synthetic?: false
         Parameter class: interface java.util.Collection
          Parameter name: c
               Modifiers: 0
            Is implicit?: false
        Is name present?: true
           Is synthetic?: false
                                 
# 方法四
public boolean ExampleMethods.simpleMethod(java.lang.String,int)
             Return type: boolean
     Generic return type: boolean
         Parameter class: class java.lang.String
          Parameter name: stringParam
               Modifiers: 0
            Is implicit?: false
        Is name present?: true
           Is synthetic?: false
         Parameter class: int
          Parameter name: intParam
               Modifiers: 0
            Is implicit?: false
        Is name present?: true
           Is synthetic?: false

这样就把参数名给打印出来了，Is name present为true。

出于好奇，以十六进制打开ExampleMethods的class文件，maven项目在idea中build出来的class不管有没有带-parameters都会把参数名编译进去，但是多了MethodParameters这几个字眼。如下图：

然后尝试直接用javac -parameters编译，打开后

很明显是没有把参数名编译进去的。

好像找不到idea执行build的时候执行了什么，所有我猜测控制java.lang.reflect.Parameter.getName()返回是否真实的参数名就是在于MethodParameters这词，在jvm加载class时识别是否有MethodParameters，而决定是否加载参数名。

这仅是猜测，望大家相告是其真正的原理。
文中也可能存在错误，也望大家指出。

上述代码全部来自#参考资料中的Obtaining Names of Method Parameters

Obtaining Names of Method Parameters