为什么在不可变类中的getter中制作防御性副本？ [英] Why make defensive copies in getters inside immutable classes?

问题描述

这个问题是关于良好的编程实践和避免潜在的漏洞。

我读了Joshua Bloch的Effective Java，这就是我想知道的：

我为什么要考虑制作防御性副本我的不可变类中的getter方法中没有mutator吗？

第二种：除了 private 之外，为什么我要创建 final 字段？这只是关于性能（不是安全性）吗？

This question is about good programming practices and avoiding potential holes.
I read Joshua Bloch's Effective Java and here is what I wonder:
Why should I consider making defensive copies in getter methods in my immutable class with no mutators in it?
And second: why should I make my fields final in addition to private ? Is this only about performance (not security) ?

推荐答案

我认为这是证明这些陈述合理的情况：

I believe this is the case that justifies this statements:

public class Immutable {

    private final String name;

    private Date dateOfBirth;

    public Immutable(String name, Date dateOfBirth) {
        this.name = name;
        this.dateOfBirth = dateOfBirth;
    }

    public String getName() {
        return name;
    }

    public Date getDateOfBirth() {
        return dateOfBirth;
    }

}

getName （）很好，因为它也返回不可变对象。但是， getDateOfBirth（）方法可能会破坏不变性，因为客户端代码可以修改返回的对象，因此将不可变对象修改为好吧：

getName() is fine as it returns immutable object as well. However the getDateOfBirth() method can break immutability because the client code can modify returned object, hence modifying the Immutable object as well:

Immutable imm = new Immutable("John", new Date());

imm.getName(); //safe
Date dateOfBirth = imm.getDateOfBirth();
//hundreds of lines later
dateOfBirth.setTime(0);  //we just modified `imm` object

返回不可变对象和基元是安全的（因为它们按价值返回）。但是你需要制作可变对象的防御性副本，例如日期：

It is safe to return immutable objects and primitives (as they are returned by value). However you need to make defensive copies of mutable objects, like Date:

public Date getDateOfBirth() {
    return new Date(dateOfBirth.getTime());
}

并将集合包装在不可变视图中（如果它们是可变的），例如请参阅 收藏集.unmodifiableList（） ：

and wrap collections in immutable views (if they are mutable), e.g. see Collections.unmodifiableList():

public List<Integer> getSomeIds() {
    return Collections.unmodifiableList(someIds);
}

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