首页 \ 问答 \ 杰克逊：将非初始化的集合字段序列化为空(Jackson: serialize non-initialized collection field as empty)

杰克逊：将非初始化的集合字段序列化为空(Jackson: serialize non-initialized collection field as empty)

 我有一个带有字段或属性的POJO，包含对象集合，如下所示：  
public class Box {

    public List<Items> items;

}
 
 默认情况下， items值为null， 我不想用空列表初始化它 。  
 现在，如果我尝试用Jackson序列化它，我会得到NullPointerException 。 有没有一种简单的方法可以让杰克逊不打破这种价值并将其序列化为空集合： [ ] ？  
 注意。 这个类只是一个简化的例子。 实际上，每个类中都有一百个类和许多具有不同名称的字段，有时在代码中的某处设置为null ，从而破坏运行时的序列化。 

I have a POJO with a field or property, containing collection of objects, something like this: 
public class Box {

    public List<Items> items;

}
 
By default, value of items is null, and I do not want to initialize it with empty list.  
Now, if I try to serialize it with Jackson, I get NullPointerException. Is there a simple way to make Jackson not break on such value and serialize it as an empty collection: [ ]? 
Note. This class is just a simplified example. In reality, there are a hundred of classes and a number of fields with different names in each of them, which are occasionally set to null sometimes somewhere in the code, breaking serialization in runtime.

原文：https://stackoverflow.com/questions/35432133

更新时间：2023-12-28 12:12

最满意答案

 问题是由数据框的stringsAsFactors参数默认为TRUE。 首次声明数据帧时，将此参数设置为FALSE，并且rbind（）函数将无错误地执行。  
#Declare some initial vectors:
students = c("Mary", "Jane", "Eva","Leon")
testScores = c(80, 90, 70, 95)
birthDates = c("1990-08-01","1995-07-23","1993-12-13","1990-08-09")
 
 声明您的数据框，并将stringsAsFactors参数设置为FALSE：  
df1 <- data.frame (students ,testScores ,birthDates , stringsAsFactors = FALSE)
df1 <- rbind(df1, c("Mike", 87, "2014-02-23"))
 
 现在新行应该附加到数据框df1：  
 
  DF1 [5,1]  
 
 结果是：  
students test1      birth
5     Mike    87 2014-02-23

The issue results from the data frame's stringsAsFactors parameter being TRUE by default. When you first declare your data frame, set this parameter to FALSE, and the rbind() function will execute without error. 
#Declare some initial vectors:
students = c("Mary", "Jane", "Eva","Leon")
testScores = c(80, 90, 70, 95)
birthDates = c("1990-08-01","1995-07-23","1993-12-13","1990-08-09")
 
Declare your data frame, and set the stringsAsFactors parameter to FALSE: 
df1 <- data.frame (students ,testScores ,birthDates , stringsAsFactors = FALSE)
df1 <- rbind(df1, c("Mike", 87, "2014-02-23"))
 
The new row should now be attached to the data frame df1: 
 
 df1[5,] 
 
results in: 
students test1      birth
5     Mike    87 2014-02-23

杰克逊：将非初始化的集合字段序列化为空(Jackson: serialize non-initialized collection field as empty)

最满意答案

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