为什么在不可变类中的getter中制作防御性副本？

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：

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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