基于ParameterizedType实现泛型类类型参数化
在上一篇中我们用到了这个方法：
public static <T> T getInstance(Object object, int i) {
if (object != null) {
return (T) ((ParameterizedType) object.getClass()
.getGenericSuperclass())
.getActualTypeArguments()[i];
}
return null;
}
在这篇中就来分析一下什么是ParameterizedType，它有什么作用。
ParameterizedType是 JDK7 开始, java.lang.reflect 提供了Interface ParameterizedType. 通过实现这个接口, 我们可以实现泛型类的类型的参数化。
先来看看它的源码：
/**
* ParameterizedType represents a parameterized type such as
* Collection<String>.
*
* <p>A parameterized type is created the first time it is needed by a
* reflective method, as specified in this package. When a
* parameterized type p is created, the generic type declaration that
* p instantiates is resolved, and all type arguments of p are created
* recursively. See {@link java.lang.reflect.TypeVariable
* TypeVariable} for details on the creation process for type
* variables. Repeated creation of a parameterized type has no effect.
*
* <p>Instances of classes that implement this interface must implement
* an equals() method that equates any two instances that share the
* same generic type declaration and have equal type parameters.
*
* @since 1.5
*/
public interface ParameterizedType extends Type {
/**
* Returns an array of {@code Type} objects representing the actual type
* arguments to this type.
*
* <p>Note that in some cases, the returned array be empty. This can occur
* if this type represents a non-parameterized type nested within
* a parameterized type.
*
* @return an array of {@code Type} objects representing the actual type
* arguments to this type
* @throws TypeNotPresentException if any of the
* actual type arguments refers to a non-existent type declaration
* @throws MalformedParameterizedTypeException if any of the
* actual type parameters refer to a parameterized type that cannot
* be instantiated for any reason
* @since 1.5
*/
Type[] getActualTypeArguments();
/**
* Returns the {@code Type} object representing the class or interface
* that declared this type.
*
* @return the {@code Type} object representing the class or interface
* that declared this type
* @since 1.5
*/
Type getRawType();
/**
* Returns a {@code Type} object representing the type that this type
* is a member of. For example, if this type is {@code O<T>.I<S>},
* return a representation of {@code O<T>}.
*
* <p>If this type is a top-level type, {@code null} is returned.
*
* @return a {@code Type} object representing the type that
* this type is a member of. If this type is a top-level type,
* {@code null} is returned
* @throws TypeNotPresentException if the owner type
* refers to a non-existent type declaration
* @throws MalformedParameterizedTypeException if the owner type
* refers to a parameterized type that cannot be instantiated
* for any reason
* @since 1.5
*/
Type getOwnerType();
}
- 发现ParameterizedType是一个接口，并继承Type这个对象，Type是什么呢我们点进去也看看：
/**
* Type is the common superinterface for all types in the Java
* programming language. These include raw types, parameterized types,
* array types, type variables and primitive types.
*
* @since 1.5
*/
public interface Type {
/**
* Returns a string describing this type, including information
* about any type parameters.
*
* @implSpec The default implementation calls {@code toString}.
*
* @return a string describing this type
* @since 1.8
*/
default String getTypeName() {
return toString();
}
}
这是官方的注释:
Type is the common superinterface for all types in the Java programming language. These include raw types, parameterized types, array types, type variables and primitive types.
Type 是Java编程语言中所有类型的通用超级接口。这些类型包括原始类型、参数化类型、数组类型、类型变量和基元类型。
这里解释的很清楚了，Type接口是Java编程语言中所有类型的通用超级接口。
下面我们继续看ParameterizedType：
A parameterized type is created the first time it is needed by a reflective method, as specified in this package. When a parameterized type p is created, the generic type declaration that p instantiates is resolved, and all type arguments of p are created
recursively. See {@link java.lang.reflect.TypeVariable TypeVariable} for details on the creation process for type variables.
Repeated creation of a parameterized type has no effect.
大概意思是说：ParameterizedType是在反射方法第一次需要时创建的，如包中指定的。当创建参数化类型p时，解析p实例化的泛型类型声明，并且递归地创建p的所有类型参数。有关类型变量创建过程的详细信息，请参见@link java.lang.reflect.typevariable typevariable。
重复创建参数化类型没有效果。
怎么来判断参数化类型呢？
通过下面例子来说明
public class ParameterizedTypeBean {
List<String> listString;
List list;
Map<String,Long> mapString;
Map map;
Map.Entry<Long,Short> mapLong;
}
测试：
Field[] declaredFields = ParameterizedTypeBean.class.getDeclaredFields(); // 反射获取字段
for (Field field : declaredFields) {
// 是否是 ParameterizedType 参数化类型
System.out.println(field.getName() + ": " + (field.getGenericType() instanceof ParameterizedType));
}
结果打印：
listString: true
list: false
mapString: true
map: false
mapLong: true
从打印结果看来,具有<>（泛型）符号的变量是参数化类型。
我们分别来看ParameterizedType内部的三个方法：
该方法返回一个Type数组
Type[] getActualTypeArguments();
测试代码：
Field[] declaredFields = ParameterizedTypeBean.class.getDeclaredFields(); // 反射获取字段
for (Field field : declaredFields) {
if (field.getGenericType() instanceof ParameterizedType) {
ParameterizedType genericType = (ParameterizedType) field.getGenericType();
System.out.print("变量类型 = " + genericType.getTypeName() + " ");
System.out.print("变量名称 = " + field.getName() + " ");
// 泛型类型的数组
Type[] actualTypeArguments = genericType.getActualTypeArguments();
for (Type type: actualTypeArguments) {
System.out.print("泛型类型 = " + type.getTypeName() + " ");
}
System.out.println();
System.out.println();
}
}
打印结果：
变量类型 = java.util.List<java.lang.String> 变量名称 = listString 泛型类型 = java.lang.String
变量类型 = java.util.Map<java.lang.String, java.lang.Long> 变量名称 = mapString 泛型类型 = java.lang.String 泛型类型 = java.lang.Long
变量类型 = java.util.Map$Entry<java.lang.Long, java.lang.Short> 变量名称 = mapLong 泛型类型 = java.lang.Long 泛型类型 = java.lang.Short
ps: 从打印结果返回来看,getActualTypeArguments()返回了一个Type数组,数组里是参数化类型的参数。
Type getRawType();
测试代码：
Field[] declaredFields = ParameterizedTypeBean.class.getDeclaredFields(); // 反射获取字段
for (Field field : declaredFields) {
if (field.getGenericType() instanceof ParameterizedType) {
ParameterizedType genericType = (ParameterizedType) field.getGenericType();
System.out.print("变量 名称= " + field.getName() + " ");
// 获取变量的类型 genericType.getRawType()
System.out.print("getRawType() = " + genericType.getRawType().getTypeName());
System.out.println();
System.out.println();
}
}
打印结果：
变量 名称= listString getRawType() = java.util.List
变量 名称= mapString getRawType() = java.util.Map
变量 名称= mapLong getRawType() = java.util.Map$Entry
PS:从打印结果来看,getRawType()是变量的类型
Type getOwnerType();
测试代码：
Field[] declaredFields = ParameterizedTypeBean.class.getDeclaredFields(); // 反射获取字段
for (Field field : declaredFields) {
if (field.getGenericType() instanceof ParameterizedType) {
ParameterizedType genericType = (ParameterizedType) field.getGenericType();
System.out.print("变量 名称= " + field.getName() + " ");
Type ownerType = genericType.getOwnerType();
if (ownerType != null) {
System.out.print("getOwnerType() = " + ownerType.getTypeName());
} else {
System.out.print("getOwnerType() = null");
}
System.out.println();
System.out.println();
}
}
打印结果：
变量 名称= listString getOwnerType() = null
变量 名称= mapString getOwnerType() = null
变量 名称= mapLong getOwnerType() = java.util.Map
PS：从打印结果来看,前面两个都为null,最后一个为Map类型
前两个为什么会返回 null 呢。看一下官网的注释：
Returns a {@code Type} object representing the type that this type is a member of. For example, if this type is {@code O<T>.I<S>}, return a representation of {@code O<T>}.
大概意思是说：返回@code type对象，该对象表示此类型所属的类型。例如，如果此类型为@code o<t>.i<s>，则返回@code o<t>的表示形式。
也就是说：
O<T>.I<S>类型的变量,调用getOwnerType()会返回O<T>
官网还有一句：
If this type is a top-level type, {@code null} is returned.
如果此类型是顶级类型，则返回@code null。
这就解释为什么会打印 null 了。
总结：
1.Type 是Java编程语言中所有类型的通用超级接口。这些类型包括原始类型、参数化类型、数组类型、类型变量和基元类型。
- 内部的 getTypeName() 方法会返回对应类型的全限定类名。
2.只有具有<>（泛型）符号的变量是参数化类型。
3.getActualTypeArguments()返回了一个Type数组,数组里是参数化类型的参数，可以根据这个方法获取到 “泛型的类型”。
4.getRawType()是获取变量的类型。
5.getOwnerType()方法对于O<T>.I<S>类型的变量,会返回O<T>，例如：Map.Entry<Long,Short> mapLong，会返回java.util.Map，而不是java.util.Entry。
基于ParameterizedType获取泛型参数类型的实例：https://www.jianshu.com/p/5a7f36c7dd9e
