JsonMappingException：找不到类型[simple type，class]的合适构造函数：无法从JSON对象实例化

本文翻译自：JsonMappingException: No suitable constructor found for type [simple type, class ]: can not instantiate from JSON object

I am getting the following error when trying to get a JSON request and process it: 我在尝试获取JSON请求并处理它时收到以下错误：

org.codehaus.jackson.map.JsonMappingException: No suitable constructor found for type [simple type, class com.myweb.ApplesDO]: can not instantiate from JSON object (need to add/enable type information?) org.codehaus.jackson.map.JsonMappingException：找不到类型[simple type，class com.myweb.ApplesDO]的合适构造函数：无法从JSON对象实例化（需要添加/启用类型信息？）

Here is the JSON I am trying to send: 这是我要发送的JSON：

{"applesDO" : [{"apple" : "Green Apple"},{"apple" : "Red Apple"}]
}

In Controller, I have the following method signature: 在Controller中，我有以下方法签名：

@RequestMapping("showApples.do")
public String getApples(@RequestBody final AllApplesDO applesRequest){// Method Code
}

AllApplesDO is a wrapper of ApplesDO : AllApplesDO是ApplesDO的包装器：

public class AllApplesDO {private List<ApplesDO> applesDO;public List<ApplesDO> getApplesDO() {return applesDO;}public void setApplesDO(List<ApplesDO> applesDO) {this.applesDO = applesDO;}
}

ApplesDO: ApplesDO：

public class ApplesDO {private String apple;public String getApple() {return apple;}public void setApple(String appl) {this.apple = apple;}public ApplesDO(CustomType custom){//constructor Code}
}

I think that Jackson is unable to convert JSON into Java objects for subclasses. 我认为Jackson无法将JSON转换为子类的Java对象。 Please help with the configuration parameters for Jackson to convert JSON into Java Objects. 请帮助Jackson将JSON转换为Java Objects的配置参数。 I am using Spring Framework. 我正在使用Spring Framework。

EDIT: Included the major bug that is causing this problem in the above sample class - Please look accepted answer for solution. 编辑：包括在上面的示例类中导致此问题的主要错误 - 请查看接受的解决方案的答案。

#1楼

参考：https://stackoom.com/question/VzoV/JsonMappingException-找不到类型-simple-type-class-的合适构造函数-无法从JSON对象实例化

#2楼

This happens for these reasons: 出现这种情况的原因如下：

your inner class should be defined as static 你的内部类应该定义为静态
```
 private static class Condition { //jackson specific } 
```
It might be that you got no default constructor in your class ( UPDATE: This seems not to be the case) 可能是你的类中没有默认构造函数（ 更新：似乎不是这种情况）
```
 private static class Condition { private Long id; public Condition() { } // Setters and Getters } 
```
It could be your Setters are not defined properly or are not visible (eg private setter) 可能是您的Setter未正确定义或不可见（例如私人制定者）

#3楼

I would like to add another solution to this that does not require a dummy constructor. 我想为此添加另一个不需要虚拟构造函数的解决方案。 Since dummy constructors are a bit messy and subsequently confusing. 因为虚拟构造函数有点混乱并且随后令人困惑。 We can provide a safe constructor and by annotating the constructor arguments we allow jackson to determine the mapping between constructor parameter and field. 我们可以提供一个安全的构造函数，并通过注释构造函数参数，我们允许jackson确定构造函数参数和字段之间的映射。

so the following will also work. 所以以下也会奏效。 Note the string inside the annotation must match the field name. 请注意，注释中的字符串必须与字段名称匹配。

import com.fasterxml.jackson.annotation.JsonProperty;
public class ApplesDO {private String apple;public String getApple() {return apple;}public void setApple(String apple) {this.apple = apple;}public ApplesDO(CustomType custom){//constructor Code}public ApplesDO(@JsonProperty("apple")String apple) {}}

#4楼

Thumb Rule : Add a default constructor for each class you used as a mapping class. Thumb Rule ：为您用作映射类的每个类添加默认构造函数。 You missed this and issue arise! 你错过了这个，问题就出现了！
Simply add default constructor and it should work. 只需添加默认构造函数即可。

#5楼

When I ran into this problem, it was a result of trying to use an inner class to serve as the DO. 当我遇到这个问题时，它是尝试使用内部类作为DO的结果。 Construction of the inner class (silently) required an instance of the enclosing class -- which wasn't available to Jackson. 构建内部类（默默地）需要一个封闭类的实例 - 杰克逊无法获得。

In this case, moving the inner class to its own .java file fixed the problem. 在这种情况下，将内部类移动到其自己的.java文件可以解决问题。

#6楼

Failing custom jackson Serializers/Deserializers could also be the problem. 失败的自定义jackson序列化器/解串器也可能是问题所在。 Though it's not your case, it's worth mentioning. 虽然不是你的情况，但值得一提。

I faced the same exception and that was the case. 我遇到了同样的例外情况。

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JsonMappingException：找不到类型[simple type，class]的合适构造函数：无法从JSON对象实例化

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