XmlElement注解在Java列表属性：要注意实例化，否则从xml数据反序列化到Java对象会失败

听海边涛声2023-12-03 10:22

例如，下面的代码，Java类Conditions的属性conditionList是一个列表类型。XmlRootElement注解在conditionList的get方法上（等同于注解在属性上）。需要注意该属性的实例化，如果不实例化的话，从xml数据反序列化到Java对象的时候出错。

可以在两个地方实例化：

在 get方法中，首先判断conditionList是否为null，如果为null的话，则实例化：

package com.thb.platform;

import java.util.ArrayList;
import java.util.List;

import jakarta.xml.bind.annotation.XmlAttribute;
import jakarta.xml.bind.annotation.XmlElement;
import jakarta.xml.bind.annotation.XmlRootElement;
import jakarta.xml.bind.annotation.XmlType;
import jakarta.xml.bind.annotation.XmlValue;

@XmlRootElement(name = "conditions")
@XmlType(propOrder = {"identify", "table", "conditionList", "total", "pageSize", "pageNo"})
public class Conditions {
...省略
private List<Conditions.Condition> conditionList;
复制代码
```
  @XmlElement(name = "condition")
  public List<Conditions.Condition> getconditionList(){
      if (conditionList == null) {
          conditionList = new ArrayList<>();
      }
      return this.conditionList;
  }
  
  public void setCondition( List<Conditions.Condition> conditionList) {
      this.conditionList = conditionList;
  }
```
...省略
复制代码
```
  @XmlType(propOrder = {"value"})
  public static class Condition {
```
...省略
}
}
在声明属性conditionList的时候实例化：

package com.thb.platform;

import java.util.ArrayList;
import java.util.List;

import jakarta.xml.bind.annotation.XmlAttribute;
import jakarta.xml.bind.annotation.XmlElement;
import jakarta.xml.bind.annotation.XmlRootElement;
import jakarta.xml.bind.annotation.XmlType;
import jakarta.xml.bind.annotation.XmlValue;

@XmlRootElement(name = "conditions")
@XmlType(propOrder = {"identify", "table", "conditionList", "total", "pageSize", "pageNo"})
public class Conditions {
...省略
复制代码
```
  private List<Conditions.Condition> conditionList = new ArrayList<>();

  @XmlElement(name = "condition")
  public List<Conditions.Condition> getconditionList(){
      return this.conditionList;
  }
  
  public void setCondition( List<Conditions.Condition> conditionList) {
      this.conditionList = conditionList;
  }
```
...省略
复制代码
```
  @XmlType(propOrder = {"value"})
  public static class Condition {
```
...省略
}
}