java/w8/GenericReflection.java

import java.lang.reflect.Field;
import java.lang.reflect.ParameterizedType;
import java.lang.reflect.Type;
import java.util.Map;

public class GenericReflection {
    public static void main(String[] args) {
        System.out.println("========== Java泛型擦除后通过反射获取泛型信息 ==========\n");

        System.out.println("1. 获取字段的泛型类型:");
        try {
            Field mapField = Cache.class.getDeclaredField("cache");
            System.out.println("   字段名: " + mapField.getName());
            System.out.println("   原始类型: " + mapField.getType());

            Type genericType = mapField.getGenericType();
            if (genericType instanceof ParameterizedType) {
                ParameterizedType pt = (ParameterizedType) genericType;
                System.out.println("   参数化类型: " + pt.getTypeName());
                System.out.println("   泛型参数[0]: " + pt.getActualTypeArguments()[0]);
                System.out.println("   泛型参数[1]: " + pt.getActualTypeArguments()[1]);
            }
        } catch (NoSuchFieldException e) {
            e.printStackTrace();
        }

        System.out.println("\n2. 获取子类的泛型类型(通过ParameterizedType):");
        StringCache cache = new StringCache();
        cache.put("test", "value");

        Type superType = cache.getClass().getGenericSuperclass();
        if (superType instanceof ParameterizedType) {
            ParameterizedType pt = (ParameterizedType) superType;
            System.out.println("   父类类型: " + pt.getTypeName());
            System.out.println("   K类型: " + pt.getActualTypeArguments()[0]);
            System.out.println("   V类型: " + pt.getActualTypeArguments()[1]);
        }

        System.out.println("\n3. 总结:");
        System.out.println("   - 泛型信息在编译时被擦除到上限类型(Object或指定上限)");
        System.out.println("   - 通过反射可以在运行时获取以下泛型信息:");
        System.out.println("     * Field.getGenericType() 获取字段的泛型类型");
        System.out.println("     * Method.getGenericParameterTypes() 获取方法参数的泛型类型");
        System.out.println("     * Method.getGenericReturnType() 获取方法返回值的泛型类型");
        System.out.println("     * Class.getGenericSuperclass() 获取父类的泛型类型");
        System.out.println("   - 子类继承泛型类时，泛型信息会保留在子类的Class对象中");
    }
}

class StringCache extends Cache<String, String> {
    public StringCache() {
        super(10);
    }
}