Java获取泛型T的类型 T.class-蒲公英云

Java获取泛型T的类型 T.class

import java.lang.reflect.ParameterizedType;
import java.lang.reflect.Type;
public class Main{
    public static void main(String[] args)
    {
        Foo<String> foo = new Foo<String>(){};
        // 在类的外部这样获取
        Type type = ((ParameterizedType)foo.getClass().getGenericSuperclass()).getActualTypeArguments()[0];
        System.out.println(type);
        // 在类的内部这样获取
        System.out.println(foo.getTClass());
    }
}
abstract class Foo<T>{
    public Class<T> getTClass()
    {
        Class<T> tClass = (Class<T>)((ParameterizedType)getClass().getGenericSuperclass()).getActualTypeArguments()[0];
        return tClass;
    }
}

输出:

class java.lang.String
class java.lang.String

上面的代码不是万能的,只有实例化T的子类才能按上述方法获得T的实际类型,
如果子类没有实例化T,则无法获取T的实际类型;
比如,class Child 并没有实例化T,所以得不到String.class;

import java.lang.reflect.ParameterizedType;
import java.lang.reflect.Type;
public class Main{
    public static void main(String[] args){
        //区别在new Child<String>()没有{}匿名类
        Foo<String> foo = new Child<String>();
        // 在类的外部这样获取
        Type type = ((ParameterizedType)foo.getClass().getGenericSuperclass()).getActualTypeArguments()[0];
        System.out.println(type);
        // 在类的内部这样获取
        System.out.println(foo.getTClass());
    }
}
abstract class Foo<T>{
    public Class<T> getTClass()
    {
        Class<T> tClass = (Class<T>)((ParameterizedType)getClass().getGenericSuperclass()).getActualTypeArguments()[0];
        return tClass;
    }
}
class Child<T> extends Foo<T>{
}

输出：

Exception in thread "main" java.lang.ClassCastException: java.lang.Class cannot be cast to java.lang.reflect.ParameterizedType
    at com.hankcs.Main.main(Main.java:9)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:606)

有一种解决方式,父类本身不获取泛型的具体类型,仅提供抽象方法,由子类来提供具体的类型

public abstract class Foo<T> {  
    public abstract Class getEntityClass();  
}  
public class Child extends Foo<String> {  
    public Class getEntityClass() {  
        return String.class;  
    }  
}

对于获取泛型的方法,比较完整的代码如下:

import java.lang.reflect.ParameterizedType;
import java.lang.reflect.Type;
public class GenericsUtils {
    /**
     * 通过反射,获得定义Class时声明的父类的范型参数的类型. 如public BookManager extends
     * GenricManager<Book>
     * 
     * @param clazz The class to introspect
     * @return the first generic declaration, or <code>Object.class</code> if cannot be determined
     */
    public static Class getSuperClassGenricType(Class clazz) {
        return getSuperClassGenricType(clazz, 0);
    }
    /**
     * 通过反射,获得定义Class时声明的父类的范型参数的类型. 如public BookManager extends GenricManager<Book>
     * 
     * @param clazz clazz The class to introspect
     * @param index the Index of the generic ddeclaration,start from 0.
     */
    public static Class getSuperClassGenricType(Class clazz, int index)
            throws IndexOutOfBoundsException {
        Type genType = clazz.getGenericSuperclass();
        if (!(genType instanceof ParameterizedType)) {
            return Object.class;
        }
        Type[] params = ((ParameterizedType) genType).getActualTypeArguments();
        if (index >= params.length || index < 0) {
            return Object.class;
        }
        if (!(params[index] instanceof Class)) {
            return Object.class;
        }
        return (Class) params[index];
    }
}

转载来源:
http://blog.csdn.net/wangjunjun2008/article/details/43970217
http://www.hankcs.com/program/t-class.html
https://www.cnblogs.com/sirab415/p/6133533.html

转载于：https://blog.csdn.net/changsa65/article/details/78790881