Java获取泛型T的类型 T.class

旧城等待， 2022-05-18 05:12 303阅读 0赞

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_blog.csdn.net_wangjunjun2008_article_details_43970217]   
[http://www.hankcs.com/program/t-class.html][http_www.hankcs.com_program_t-class.html]   
[https://www.cnblogs.com/sirab415/p/6133533.html][https_www.cnblogs.com_sirab415_p_6133533.html]

转载于：[https://blog.csdn.net/changsa65/article/details/78790881][https_blog.csdn.net_changsa65_article_details_78790881]

[http_blog.csdn.net_wangjunjun2008_article_details_43970217]: http://blog.csdn.net/wangjunjun2008/article/details/43970217
[http_www.hankcs.com_program_t-class.html]: http://www.hankcs.com/program/t-class.html
[https_www.cnblogs.com_sirab415_p_6133533.html]: https://www.cnblogs.com/sirab415/p/6133533.html
[https_blog.csdn.net_changsa65_article_details_78790881]: https://blog.csdn.net/changsa65/article/details/78790881