前言

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。

正文

代码

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

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。

带-parameters

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。

留个问题

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。

代码来源

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

参考资料