Java Type技术笔录
一、吐槽
想必很多人都玩过Java反射，对于类而言大家都有不同的认知和理解，那么什么是Type呢？
二、UML
Type的演化
- java.lang.reflect
Type：所有类型的基础接口；
ParameterizedType：参数化泛型接口，如List
-> 实现类: ParameterizedTypeImpl
TypeVariable：自定义泛型接口，如List
-> 实现类: TypeVariableImpl
WildcardType：通配符泛型接口，如List、List等中的?，没有明确指明范型类型；
-> 实现类: WildcardTypeImpl
GenericArrayType：数组泛型接口，如List
-> 实现类: GenericArrayTypeImpl
- java.lang
Class: 实现了Type接口，已家喻户晓，^ ^。
三、常用获取Type API
- java.lang.Class
1、Type[] getGenericInterfaces()
描述: 返回值是Type数组，Type类型为Class或ParameterizedType子类类型，取决于实现的接口指定的泛型；
2、Type getGenericSuperclass()
描述: 返回值是Type，Type类型为Class或ParameterizedType子类类型，取决于实现的接口指定的泛型；
3、TypeVariable
描述: 返回值是类型变量数组，当定义的Class是泛型类时数组才有对应的类型变量元素，否则返回长度为0的数组；
- java.lang.reflect.Field
Type getGenericType()
描述: 返回值是Type，Type类型可为Class、ParameterizedType(如:成员变量 List
- java.lang.reflect.Method
1、Type[] getGenericParameterTypes()
描述: 返回值是Type[]，Type类型可为Class、ParameterizedType(如:成员变量 List
2、Type getGenericReturnType()
描述: 返回值是Type[]，Type类型可为Class、ParameterizedType(如:成员变量 List
3、Type[] getGenericExceptionTypes()
描述: 返回值是Type[]，Type类型可为Class、TypeVariable(如:成员变量 T ，T extends Throwable)
四、ParameterizedType
常见调用关系
API:
1、Type[] getActualTypeArguments()
描述: 获取泛型参数的实际参数类型，包含已知全部Type类型: Class、ParameterizeType、TypeVariable、WildcardType和GenericArrayType
2、Type getRawType()
描述：获取原始ParameterizedType实际的Class类型
3、Type getOwnerType()
描述: 获取外部类的类型，如果没有外部类返回Null
代码样例:
public class ParameterizedTypeDemo {
private static Interface mInterface;
private static Interface genericMethod(Interface arg) {
return null;
}
public static void main(String[] args) throws Throwable {
// RealClassImpl实现的接口是否 “ParameterizedType”
System.out.println("RealClassImpl's genericInterface is ParameterizedType: "
+ ParameterizedType.class.isInstance(RealClassImpl.class.getGenericInterfaces()[0]));
// RealClassImpl的父类是否 “ParameterizedType”
System.out.println("RealClassImpl's genericSuperclass is ParameterizedType: "
+ ParameterizedType.class.isInstance(RealClassImpl.class.getGenericSuperclass()));
// mInterface静态成员变量映射的Field字段获取其对应的类型是否 “ParameterizedType”
Field field = ParameterizedTypeDemo.class.getDeclaredField("mInterface");
System.out.println(
"mInterface is ParameterizedType: " + ParameterizedType.class.isInstance(field.getGenericType()));
// genericMethod静态成员方法对应的 “参数” 和 “返回值” 类型是否 “ParameterizedType”
Method method = ParameterizedTypeDemo.class.getDeclaredMethod("genericMethod", Interface.class);
System.out.println("genericMethod has ParameterizedType parameters: "
+ ParameterizedType.class.isInstance(method.getGenericParameterTypes()[0]));
System.out.println("genericMethod has ParameterizedType return: "
+ ParameterizedType.class.isInstance(method.getGenericReturnType()));
// RealClassImpl的父类在继承时声明 “参数化类型” 时，对应其内部的泛型参数是否也是 “ParameterizedType”
Type type = ((ParameterizedType) RealClassImpl.class.getGenericSuperclass()).getActualTypeArguments()[0];
System.out.println("RealClassImpl's genericSuperclass parameter is ParameterizedType:"
+ ParameterizedType.class.isInstance(type));
// RealClassImpl实现的接口在指定其参数为 “泛型数组类型” 时，对应泛型数组的内部元素组件类型是否 “ParameterizedType”
type = ((ParameterizedType) RealClassImpl.class.getGenericInterfaces()[0]).getActualTypeArguments()[0];
System.out.println("RealClassImpl's genericInterface parameter is GenericArrayType:"
+ GenericArrayType.class.isInstance(type));
System.out.println(
"RealClassImpl's genericInterface parameter is GenericArrayType, and genericComponentType inside GenericArrayType is ParameterizedType:"
+ ParameterizedType.class.isInstance(((GenericArrayType) type).getGenericComponentType()));
}
interface Interface {
}
static abstract class AbstractClass {
}
static class RealClassImpl extends AbstractClass> implements Interface[]> {
}
} 输出结果:
RealClassImpl's genericInterface is ParameterizedType: true
RealClassImpl's genericSuperclass is ParameterizedType: true
mInterface is ParameterizedType: true
genericMethod has ParameterizedType parameters: true
genericMethod has ParameterizedType return: true
RealClassImpl's genericSuperclass parameter is ParameterizedType:true
RealClassImpl's genericInterface parameter is GenericArrayType:true
RealClassImpl's genericInterface parameter is GenericArrayType, and genericComponentType inside GenericArrayType is ParameterizedType:true
五、TypeVariable
常见调用关系
API:
1、Type[] getBounds()
描述: 获取TypeVariable的边界，如定义Class A
2、D getGenericDeclaration()
描述: 获取声明泛型类型变量的Class、Method或Constructor，泛型简单声明简介如下:
Class声明泛型:
class Sample{}
Method声明泛型:
void sample(T t){}
Constructor声明泛型
sample(T t){}
代码样例:
public class TypeVariableDemo {
public static void main(String[] args) throws Throwable {
// Interface的typeParameters得到其泛型参数是否是 “TypeVariable”
System.out.println("Interface's typeParameters is only TypeVariable:"
+ TypeVariable.class.isInstance(Interface.class.getTypeParameters()[0]));
// RealClass的typeParameters得到其泛型参数是否是 “TypeVariable”
System.out.println("RealClass's typeParameters is only TypeVariable:"
+ TypeVariable.class.isInstance(RealClass.class.getTypeParameters()[0]));
// RealClass's genericMethod 对应的Method获取到的泛型参数是否是 “TypeVariable”
Method method = RealClass.class.getDeclaredMethod("genericMethod", Exception.class);
System.out.println("RealClass's genericParameters of genericMethod is only TypeVariable:"
+ TypeVariable.class.isInstance(method.getGenericParameterTypes()[0]));
// RealClass's genericMethod 对应的Method获取到的返回值泛型是否是 “TypeVariable”
System.out.println("RealClass's returnType of genericMethod is only TypeVariable:"
+ TypeVariable.class.isInstance(method.getGenericReturnType()));
// RealClass's genericMethod 对应的Method获取到的throws异常类型是否是 “TypeVariable”
System.out.println("RealClass's throw Excetpion of genericMethod is only TypeVariable:"
+ TypeVariable.class.isInstance(method.getGenericExceptionTypes()[0]));
// RealClass's mMemberField 对应的Field获取到的泛型类型是否是 “TypeVariable”
Field field = RealClass.class.getDeclaredField("mMemberField");
System.out.println("RealClass's type of mMemberField is only TypeVariable:"
+ TypeVariable.class.isInstance(field.getGenericType()));
// InterfaceImpl所实现的接口提供的泛型类型ParameterizedType的actualTypeArguments是否是 “TypeVariable”
Type type = InterfaceImpl.class.getGenericInterfaces()[0];
if (ParameterizedType.class.isInstance(type)) {
System.out.println("InterfaceImpl's implemented Inteface's GenericType is TypeVariable:"
+ TypeVariable.class.isInstance(((ParameterizedType) type).getActualTypeArguments()[0]));
}
// 在RealClass中的getBounds使用案例
System.out.println("RealClass's second genericType B getBounds:"
+ Arrays.toString(RealClass.class.getTypeParameters()[1].getBounds()));
// 在RealClass中的getGenericDeclaration使用案例
System.out.println("RealClass's second genericType B getGenericDeclaration:"
+ RealClass.class.getTypeParameters()[1].getGenericDeclaration());
System.out.println("RealClass's second genericType S from genericMethod's getGenericDeclaration:"
+ ((TypeVariable) method.getGenericReturnType()).getGenericDeclaration());
}
interface Interface {
}
static class InterfaceImpl implements Interface {
}
static class RealClass> {
T mMemberField;
S genericMethod(T t) throws T {
throw (T) new Exception();
}
}
} 输出结果:
Interface's typeParameters is only TypeVariable:true
RealClass's typeParameters is only TypeVariable:true
RealClass's genericParameters of genericMethod is only TypeVariable:true
RealClass's returnType of genericMethod is only TypeVariable:true
RealClass's throw Excetpion of genericMethod is only TypeVariable:true
RealClass's type of mMemberField is only TypeVariable:true
InterfaceImpl's implemented Inteface's GenericType is TypeVariable:true
RealClass's second genericType B getBounds:[interface java.lang.Runnable, interface java.io.Serializable, java.util.Comparator
] RealClass's second genericType B getGenericDeclaration:class TypeVariableDemo$RealClass
RealClass's second genericType S from genericMethod's getGenericDeclaration:java.lang.Object TypeVariableDemo$RealClass.genericMethod(java.lang.Exception) throws java.lang.Exception
六、WildcardType
常见调用关系
API:
1、Type[] getLowerBounds()
描述: 获取通配符类型的下限边界，如果没有使用? super，返回空数组
2、Type[] getUpperBounds()
描述: 获取通配符类型的上限边界，如果没有使用? extends，返回包含一个Object.class元素的数组
代码样例:
public class WildcardTypeDemo {
public static void main(String[] args) throws Throwable {
// 所实现的接口的内部Interface泛型指定的内部泛型类型是否是 “WildcardType”
System.out.println(
"InterfaceImpl's implemented Interface's GenericType of Inner GenericType Interface is WildcardType:"
+ WildcardType.class.isInstance(
((ParameterizedType) ((ParameterizedType) InterfaceImpl.class.getGenericInterfaces()[0])
.getActualTypeArguments()[0]).getActualTypeArguments()[0]));
// InterfaceImpl的成员方法的Method获取到的泛型参数中泛型的内部参数类型是否是 “WildcardType”
Method method = InterfaceImpl.class.getDeclaredMethod("genericMethod", Interface.class);
System.out.println("InterfaceImpl's GenericParamerters's Inner Type of genericMethod is WildcardType:"
+ WildcardType.class.isInstance(
((ParameterizedType) method.getGenericParameterTypes()[0]).getActualTypeArguments()[0]));
// InterfaceImpl的成员方法的Method获取到的返回值类型中泛型的内部参数类型是否是 “WildcardType”
System.out.println(
"InterfaceImpl's GenericReturn's Inner Type of genericMethod is WildcardType:" + WildcardType.class
.isInstance(((ParameterizedType) method.getGenericReturnType()).getActualTypeArguments()[0]));
// InterfaceImpl的成员变量的泛型的内部参数类型是否是 “WildcardType”
Field field = InterfaceImpl.class.getDeclaredField("mMememberField");
Type type = ((ParameterizedType) field.getGenericType()).getActualTypeArguments()[0];
System.out.println("InterfaceImpl's mMememberField's Inner Type of GenericType is WildcardType:"
+ WildcardType.class.isInstance(type));
// getLowerBounds和getUpperBounds使用案例
// mMememberField
System.out.println("mMememberField getLowerBounds:" + Arrays.toString(((WildcardType) type).getLowerBounds()));
System.out.println("mMememberField getUpperBounds:" + Arrays.toString(((WildcardType) type).getUpperBounds()));
// mMememberField1
field = InterfaceImpl.class.getDeclaredField("mMememberField1");
type = ((ParameterizedType) field.getGenericType()).getActualTypeArguments()[0];
System.out.println("mMememberField1 getLowerBounds:" + Arrays.toString(((WildcardType) type).getLowerBounds()));
System.out.println("mMememberField1 getUpperBounds:" + Arrays.toString(((WildcardType) type).getUpperBounds()));
}
interface Interface {
}
static class InterfaceImpl implements Interface> {
Interface> mMememberField;
Interface> mMememberField1;
Interface genericMethod(Interface f) {
return null;
}
}
} 输出结果:
InterfaceImpl's implemented Interface's GenericType of Inner GenericType Interface is WildcardType:true
InterfaceImpl's GenericParamerters's Inner Type of genericMethod is WildcardType:true
InterfaceImpl's GenericReturn's Inner Type of genericMethod is WildcardType:true
InterfaceImpl's mMememberField's Inner Type of GenericType is WildcardType:true
mMememberField getLowerBounds:[java.util.ArrayList]
mMememberField getUpperBounds:[class java.lang.Object]
mMememberField1 getLowerBounds:[]
mMememberField1 getUpperBounds:[java.util.List]
七、GenericArrayType
常见调用关系
API:
1、Type getGenericComponentType()
描述: 获取泛型类型数组，Type为TypeVariable和ParamerizeType类型
代码样例:
public class GenericArrayTypeDemo {
public static void main(String[] args) throws Throwable {
// 从RealClass实现的接口Interface中提取出泛型类型是否是 “GenericArrayType”
System.out
.println("RealClass's implemented Interface's Inner GenericType of its GenericType is GenericArrayType:"
+ GenericArrayType.class
.isInstance(((ParameterizedType) RealClass.class.getGenericInterfaces()[0])
.getActualTypeArguments()[0]));
// 从RealClass中的genericMethod的Method获取到的泛型参数是否是 “GenericArrayType”
Method method = RealClass.class.getDeclaredMethod("genericMethod", Object[].class);
System.out.println("genericMethod's Generic parameterType of its Method is GenericArrayType:"
+ GenericArrayType.class.isInstance(method.getGenericParameterTypes()[0]));
// 从RealClass中的genericMethod的Method获取到的返回值类型是否是 “GenericArrayType”
System.out.println("genericMethod's Generic returnType of its Method is GenericArrayType:"
+ GenericArrayType.class.isInstance(method.getGenericReturnType()));
// 从RealClass中的mMemberArray的Field获取其泛型类型是否是 “GenericArrayType”
Field field = RealClass.class.getDeclaredField("mMemberArray");
System.out.println("mMemberArray's genericType is GenericArrayType:"
+ GenericArrayType.class.isInstance(field.getGenericType()));
// getGenericComponentType使用案例
System.out.println("mMemberArray's genericComponentType is:"
+ ((GenericArrayType) field.getGenericType()).getGenericComponentType());
}
interface Interface {
}
static class RealClass implements Interface {
T[] mMemberArray;
S[] genericMethod(S[] array) {
return null;
}
}
} 输出结果:
RealClass's implemented Interface's Inner GenericType of its GenericType is GenericArrayType:true
genericMethod's Generic parameterType of its Method is GenericArrayType:true
genericMethod's Generic returnType of its Method is GenericArrayType:true
mMemberArray's genericType is GenericArrayType:true
mMemberArray's genericComponentType is:T
总结
1、Type定义了Java语言类型最原始的接口；
2、Class是类的原生类型，ParameterizeType、TypeVarible、WildcardType和GenericArrayType是泛型类型；
3、ParameterizeType可以包裹ParameterizeType，从而可以完成深层多层泛型嵌套。
4、结合GenericArrayType和ParameterizeType可以完成一些复杂的泛型类型解析，结合一些开源项目Jackson等完多层级泛型对象的转化...