java 谷歌番石榴函数界面中的@Nullable 输入触发 FindBugs 警告
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@Nullable input in Google Guava Function interface triggers FindBugs warning
提问by vitaly
The com.google.common.base.Functioninterface (from Google Guava) defines applyas:
将com.google.common.base.Function（从接口谷歌番石榴）定义apply为：
@Nullable T apply(@Nullable F input);
@Nullable T apply(@Nullable F input);
The method has the following javadoc note:
该方法具有以下 javadoc 注释：
@throws NullPointerException if {@code input} is null and this function does not accept null arguments.
@throws NullPointerException if {@code input} is null and this function does not accept null arguments.
FindBugs complains about my implementation of Function:
FindBugs 抱怨我的 Function 实现：
private static final class Example implements Function<MyBean, String> {
@Override
@Nullable
public String apply(@Nullable MyBean input) {
if (null == input) {
throw new NullPointerException();
}
return input.field;
}
}
with a high-prioritywarning:
带有高优先级警告：
NP_PARAMETER_MUST_BE_NONNULL_BUT_MARKED_AS_NULLABLE, Priority: High
input must be nonnull but is marked as nullable
This parameter is always used in a way that requires it to be nonnull, but the parameter is explicitly annotated as being Nullable. Either the use of the parameter or the annotation is wrong.
NP_PARAMETER_MUST_BE_NONNULL_BUT_MARKED_AS_NULLABLE，优先级：高
输入必须为非空但被标记为可为空
此参数始终以要求其为非空的方式使用，但该参数被显式注释为 Nullable。要么是参数的使用，要么是注解不对。
My function does not support nullinputs and an exception is thrown if that is the case. If I understand correctly, FindBugs treats this as a requirement for non-null.
我的函数不支持null输入，如果是这种情况会抛出异常。如果我理解正确，FindBugs 将此视为非空的要求。
To me it looks like a contradiction: input is @Nullable but method @throws NullPointerException when it is null. Am I missing something?
对我来说，它看起来像一个矛盾：输入是@Nullable 但方法 @throws NullPointerException 当它为空时。我错过了什么吗？
The only way to get rid of the warning that I can see is manual suppression. (Guava code is out of my control, obviously).
摆脱我能看到的警告的唯一方法是手动抑制。（显然，番石榴代码超出了我的控制范围）。
Who is wrong about the usage of @Nullable annotation, FindBugs, Guava or myself?
@Nullable 注释、FindBugs、Guava 或我自己的用法谁错了？
采纳答案by Xaerxess
Your implementation is wrong ;)
你的实现是错误的;)
Basically docs says (I'll paraphrase and emphasise):
基本上文档说（我会解释和强调）：
@throws NullPointerExceptionifinputis null and the concrete function implementationdoes not accept null arguments
@throws NullPointerExceptionifinput为空且具体的函数实现不接受空参数
By implementing your function you must decide if it accepts nulls or not. In first case:
通过实现您的函数，您必须决定它是否接受空值。在第一种情况下：
private static final class Example implements Function<MyBean, String> {
@Override
@Nullable
public String apply(@Nullable MyBean input) {
return input == null ? null : input.field;
}
}
In second case:
第二种情况：
private static final class Example implements Function<MyBean, String> {
@Override
@Nullable
public String apply(MyBean input) {
if (null == input) {
throw new NullPointerException();
}
return input.field;
}
}
In both examples returning null is allowed.
在这两个示例中，都允许返回 null。
EDIT:
编辑：
Note that Guava uses @javax.annotation.ParametersAreNonnullByDefaulton all packages, hence if @Nullableis present it means "suspend global @Nonnulland allow nulls here" and if not it means "nulls forbidden here".
请注意，番石榴@javax.annotation.ParametersAreNonnullByDefault在所有包上都使用，因此如果@Nullable存在，则表示“暂停全局@Nonnull并在此处允许空值”，如果不存在，则表示“此处禁止空值”。
That said, you may want use @Nonnullannotation on your argument or @ParametersAreNonnullByDefaultin package to tell FindBugs Function's argument can't be null.
也就是说，您可能希望@Nonnull在参数或@ParametersAreNonnullByDefault包中使用注释来告诉 FindBugs 函数的参数不能为空。
EDIT 2:
编辑2：
Turns out this case is known issue, see comment #3 (from Guava's lead dev Kevin Bourrillion, about his conversation with Bill Pugh, Findbugs' lead):
事实证明，此案例是已知问题，请参阅评论 #3（来自 Guava 的首席开发人员 Kevin Bourrillion，关于他与 Findbugs 的负责人 Bill Pugh 的对话）：
My reference was a series of in-person conversations with Bill Pugh. He asserted unambiguously that
@Nullablemeans only that some subtypes mightaccept null. And this seems to be borne out by findbugs for us -- our code passes the nullability checks pretty cleanly (though we should check again since this particular Function change was made).
我的参考是与 Bill Pugh 的一系列面对面对话。他毫不含糊地断言，这
@Nullable意味着只有某些子类型 可能接受 null。这似乎为我们的 findbugs 所证实——我们的代码非常干净地通过了可空性检查（尽管我们应该再次检查，因为进行了这个特定的函数更改）。
回答by Balamurugan Muthiah
Marking the parameter @Nonnullresolves the issue from findbugs.
标记参数@Nonnull可以解决 findbugs 的问题。
回答by Ben Thomas
It seems as though by default Google Guava Functions are @Nullable by default - I was getting Findbugs errors stating that "result must be nonnull but is marked as nullable" when there was no annotation. Adding @Nonnull to the function declaration in the following way helped:
似乎默认情况下，Google Guava 函数默认为 @Nullable - 当没有注释时，我收到 Findbugs 错误，指出“结果必须为非空，但被标记为可为空”。通过以下方式将 @Nonnull 添加到函数声明中有所帮助：
new Function<Object, Object>() {
@Nonnull
public Object apply(@Nonnull Object object) {
and now Findbugs is happy. Thanks all
现在 Findbugs 很高兴。谢谢大家
