为什么JAXB不为列表生成setter [英] Why doesn't JAXB generate setters for Lists

问题描述

当我从XSD生成JAXB类时，带有 maxOccurs =unbounded的元素会为它们生成一个getter方法，但没有setter方法，如下所示： / p>

When I generate JAXB classes from an XSD, the elements with maxOccurs="unbounded" gets a getter method generated for them, but no setter method, as follows:

/**
 * Gets the value of the element3 property.
 * 
 * <p>
 * This accessor method returns a reference to the live list,
 * not a snapshot. Therefore any modification you make to the
 * returned list will be present inside the JAXB object.
 * This is why there is not a <CODE>set</CODE> method for the element3 property.
 * 
 * <p>
 * For example, to add a new item, do as follows:
 * <pre>
 *    getElement3().add(newItem);
 * </pre>
 * 
 * 
 * <p>
 * Objects of the following type(s) are allowed in the list
 * {@link Type }
 * 
 * 
 */
public List<Type> getElement3() {
    if (element3 == null) {
        element3 = new ArrayList<Type>();
    }
    return this.element3;
}

方法评论清楚地告诉我如何使用它，但是我的问题如下：
为什么JAXB不按照Java Beans规则生成一个setter？我知道我自己可以编写setter方法，但建议的方法是否有任何优势在生成的getter方法中？

The method comment makes it crystal clear on how can I use it, but my question is as follows:
Why doesn't JAXB just generate a setter, following the Java Beans rules? I know I can write the setter method myself, but is there any advantage to the approach suggested in the generated getter method?

这是我的XSD：

<?xml version="1.0" encoding="UTF-8"?>
<schema xmlns="http://www.w3.org/2001/XMLSchema" xmlns:tns="http://www.example.org/DoTransfer/" targetNamespace="http://www.example.org/DoTransfer/">

    <element name="CollectionTest" type="tns:CollectionTest"></element>

    <complexType name="CollectionTest">
        <sequence>
            <element name="element1" type="string" maxOccurs="1" minOccurs="1"></element>
            <element name="element2" type="boolean" maxOccurs="1" minOccurs="1"></element>
            <element name="element3" type="tns:type" maxOccurs="unbounded" minOccurs="1" nillable="true"></element>
        </sequence>
    </complexType>


    <complexType name="type">
        <sequence>
            <element name="subelement1" type="string" maxOccurs="1" minOccurs="1"></element>
            <element name="subelement2" type="string" maxOccurs="1" minOccurs="0"></element>
        </sequence>
    </complexType>
</schema>

推荐答案

以下是JAXB规范的理由 - 第60页。

Here is the justification from the JAXB specification - page 60.

设计注意 - List属性没有setter方法。
getter通过引用返回List。可以使用java.util.List上定义的适当方法
将项添加到getter方法返回的
列表中。 JAXB 1.0中此设计的基本原理是
，以使实现能够包装列表，并且能够在列表中添加或删除内容时执行
执行检查。

因此，如果List的实现覆盖了add / remove来执行验证，那么用（例如）ArrayList替换'special'List会打败这些支票。

So if the implementation of the List was overriding add/remove to perform validation, replacing that 'special' List with (for instance) an ArrayList would defeat these checks.

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