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<T> {
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<String> listParam) {
return listParam.isEmpty();
}
public <T> void genericMethod(T[] a, Collection<T> 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<java.lang.String>)
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 <T> void ExampleMethods.genericMethod(T[],java.util.Collection<T>)
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
<build>
<plugins>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-compiler-plugin</artifactId>
<configuration>
<source>8</source>
<target>8</target>
<compilerArgument>-parameters</compilerArgument>
</configuration>
</plugin>
</plugins>
</build>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<java.lang.String>)
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 <T> void ExampleMethods.genericMethod(T[],java.util.Collection<T>)
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
