Redis缓存,泛型集合与json字符串的相互转换实例

难点是泛型如何转换

一、arrayList<Map<String, Object>>转化json字符串，存入redis缓存

    ArrayList<Map<String, Object>> listProfit 
    //将ArrayList<Map<String, Object>>类型数据转换成json字符串
 
    String listProfitPctJsonStr = JSON.toJSONString(listProfit); 
 
    //然后将json字符串存入redis缓存,唯一key value
    JedisUtils.setex("listProfit", 600,listProfitPctJsonStr);

二、json字符串转回ArrayList<Map<String, Object>>（难点）

 List<Map<String, Object>> listProfit=null;
 //先从redis里面去查询数据，看是否能获取到对应json字符串
 String jsonStrLp=JedisUtils.get("listProfit");
 
//如果能获取则说明缓存中有数据
 
if(!StringUtils.isBlank(jsonStrLp)){
 
  //目的是为了泛型的转换
 
  listProfit=new ArrayList<Map<String,Object>>();
 
  //先把从redis缓存中取出来的json字符串转为List<Map>集合
 
  List<Map> mapList=JSON.parseArray(jsonStrLp, Map.class);
 
  //然后循环遍历这个List集合，得出的结果为Map,然后再强转为Map<String,Object>,
 
     再循环 把 Map<String,Object>添加到List集合中，搞定！！！
 
  for (Map map : mapList) {
    Map<String,Object> sObj=(Map<String,Object>)map;
    listProfit.add(sObj);
}

补充知识：Java的List和Json转换以及StringRedisTemplate往redis存泛型对象

List转Json

List<User> user= new ArrayList();

String str = JSON.toJSONString(user);

Json 转List方法一

List<User> user= JSON.parseArray(json,User.class);

如果是泛型方法需要使用TypeReference

Json 转List 方法二

String json = "[{}]";

List<user> user= JSON.parseObject(json,new TypeReference<List<User>>(){});

泛型T

Json 转List方法三

List<T> students = JSON.parseObject(listCache,new TypeReference<List<T>>(){});

综合例子：Springboot环境下利用StringRedisTemplate往redis存泛型对象

一开始要注入下StringRedisTemplate

@Autowired

private StringRedisTemplate redisTemplate;

Redis获取值不存在就从数据库取出来json化存缓存，存在则直接反序列化json为List

List<T> list;
String listCache=redisTemplate.opsForValue().get(key);
if(listCache!=null){
  list = JSON.parseObject(listCache,new TypeReference<List<T>>(){});
}
else {
  list = userService.getAllList();
  redisTemplate.opsForValue().set(key, JSON.toJSONString(list), 60 * 1, TimeUnit.SECONDS);
}

附录：TypeReference源码

package com.alibaba.fastjson;
 
import java.lang.reflect.GenericArrayType;
import java.lang.reflect.ParameterizedType;
import java.lang.reflect.Type;
import java.lang.reflect.TypeVariable;
import java.util.List;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;
 
import com.alibaba.fastjson.util.ParameterizedTypeImpl;
import com.alibaba.fastjson.util.TypeUtils;
 
/** 
 * Represents a generic type {@code T}. Java doesn't yet provide a way to
 * represent generic types, so this class does. Forces clients to create a
 * subclass of this class which enables retrieval the type information even at
 * runtime.
 *
 * <p>For example, to create a type literal for {@code List<String>}, you can
 * create an empty anonymous inner class:
 *
 * <pre>
 * TypeReference&lt;List&lt;String&gt;&gt; list = new TypeReference&lt;List&lt;String&gt;&gt;() {};
 * </pre>
 * This syntax cannot be used to create type literals that have wildcard
 * parameters, such as {@code Class<?>} or {@code List<? extends CharSequence>}.
 */
public class TypeReference<T> {
  static ConcurrentMap<Type, Type> classTypeCache
      = new ConcurrentHashMap<Type, Type>(16, 0.75f, 1);
 
  protected final Type type;
 
  /**
   * Constructs a new type literal. Derives represented class from type
   * parameter.
   *
   * <p>Clients create an empty anonymous subclass. Doing so embeds the type
   * parameter in the anonymous class's type hierarchy so we can reconstitute it
   * at runtime despite erasure.
   */
  protected TypeReference(){
    Type superClass = getClass().getGenericSuperclass();
 
    Type type = ((ParameterizedType) superClass).getActualTypeArguments()[0];
 
    Type cachedType = classTypeCache.get(type);
    if (cachedType == null) {
      classTypeCache.putIfAbsent(type, type);
      cachedType = classTypeCache.get(type);
    }
 
    this.type = cachedType;
  }
 
  /**
   * @since 1.2.9
   * @param actualTypeArguments
   */
  protected TypeReference(Type... actualTypeArguments){
    Class<?> thisClass = this.getClass();
    Type superClass = thisClass.getGenericSuperclass();
 
    ParameterizedType argType = (ParameterizedType) ((ParameterizedType) superClass).getActualTypeArguments()[0];
    Type rawType = argType.getRawType();
    Type[] argTypes = argType.getActualTypeArguments();
 
    int actualIndex = 0;
    for (int i = 0; i < argTypes.length; ++i) {
      if (argTypes[i] instanceof TypeVariable &&
          actualIndex < actualTypeArguments.length) {
        argTypes[i] = actualTypeArguments[actualIndex++];
      }
      // fix for openjdk and android env
      if (argTypes[i] instanceof GenericArrayType) {
        argTypes[i] = TypeUtils.checkPrimitiveArray(
            (GenericArrayType) argTypes[i]);
      }
 
      // 如果有多层泛型且该泛型已经注明实现的情况下，判断该泛型下一层是否还有泛型
      if(argTypes[i] instanceof ParameterizedType) {
        argTypes[i] = handlerParameterizedType((ParameterizedType) argTypes[i], actualTypeArguments, actualIndex);
      }
    }
 
    Type key = new ParameterizedTypeImpl(argTypes, thisClass, rawType);
    Type cachedType = classTypeCache.get(key);
    if (cachedType == null) {
      classTypeCache.putIfAbsent(key, key);
      cachedType = classTypeCache.get(key);
    }
 
    type = cachedType;
  }
  private Type handlerParameterizedType(ParameterizedType type, Type[] actualTypeArguments, int actualIndex) {
    Class<?> thisClass = this.getClass();
    Type rawType = type.getRawType();
    Type[] argTypes = type.getActualTypeArguments();
 
    for(int i = 0; i < argTypes.length; ++i) {
      if (argTypes[i] instanceof TypeVariable && actualIndex < actualTypeArguments.length) {
        argTypes[i] = actualTypeArguments[actualIndex++];
      }
 
      // fix for openjdk and android env
      if (argTypes[i] instanceof GenericArrayType) {
        argTypes[i] = TypeUtils.checkPrimitiveArray(
            (GenericArrayType) argTypes[i]);
      }
 
      // 如果有多层泛型且该泛型已经注明实现的情况下，判断该泛型下一层是否还有泛型
      if(argTypes[i] instanceof ParameterizedType) {
        return handlerParameterizedType((ParameterizedType) argTypes[i], actualTypeArguments, actualIndex);
      }
    }
 
    Type key = new ParameterizedTypeImpl(argTypes, thisClass, rawType);
    return key;
  }
  
  /**
   * Gets underlying {@code Type} instance.
   */
  public Type getType() {
    return type;
  }
 
  public final static Type LIST_STRING = new TypeReference<List<String>>() {}.getType();
}

TypeReference的存在是因为java中子类可以获取到父类泛型的真实类型。

其中核心的方法是：getActualTypeArguments，它可以得到父类的反省类型

ParameterizedType是一个记录类型泛型的接口, 继承自Type,一共三方法:

Type[] getActualTypeArguments(); //返回泛型类型数组

Type getRawType(); //返回原始类型Type

Type getOwnerType(); //返回 Type 对象，表示此类型是其成员之一的类型。

Map<String, Integer> intMap = new HashMap<>();
    System.out.println("getSuperclass:" + intMap.getClass().getSuperclass());
    System.out.println("getGenericSuperclass:" + intMap.getClass().getGenericSuperclass());
    Type type = intMap.getClass().getGenericSuperclass();
    if (type instanceof ParameterizedType) {
      ParameterizedType p = (ParameterizedType)type;
      for (Type t : p.getActualTypeArguments()) {
        System.out.println("getActualTypeArguments>>>"+t);
      }
    }

扩展阅读Java如何获得泛型类的真实类型：

package com.paopaoedu.springboot.demo;
 
import com.paopaoedu.springboot.bean.User;
import java.lang.reflect.ParameterizedType;
import java.lang.reflect.Type;
 
public class BaseDao<T>{
 
  private Class<T> clazz;
 
  // 使用反射技术得到T的真实类型
  public Class getRealType(){
    Class c=this.getClass();
    //getSuperclass()获得该类的父类
    System.out.println("getSuperclass    >>>"+c.getSuperclass());
    //getGenericSuperclass()获得带有泛型的父类
    System.out.println("getGenericSuperclass >>>"+c.getGenericSuperclass());
    //Type是 Java 编程语言中所有类型的公共高级接口。它们包括原始类型、参数化类型、数组类型、类型变量和基本类型。
    Type type=c.getGenericSuperclass();
    System.out.println("Type         >>>"+type);
 
    //ParameterizedType参数化类型，即泛型
    // 获取当前new的对象的泛型的父类类型
    ParameterizedType pt = (ParameterizedType) this.getClass().getGenericSuperclass();
    // 获取第一个类型参数的真实类型
    this.clazz = (Class<T>) pt.getActualTypeArguments()[0];
 
    c=(Class) pt.getActualTypeArguments()[0];
    System.out.println(c);
 
    return clazz;
  }
}
 
class userdemo extends BaseDao<User>{
 
  public static void main(String[] args) {
    userdemo classB = new userdemo();
    Class realType = classB.getRealType();
    System.out.println(realType.getName());
  }
}

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