Java 不同的保留策略如何影响我的注释？
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How do different retention policies affect my annotations?
提问by xdevel2000
Can anyone explain in a clear way the practical differences between the java.lang.annotation.RetentionPolicyconstants SOURCE, CLASS, and RUNTIME?
以明确的方式有谁能解释之间的实际差别java.lang.annotation.RetentionPolicy常数SOURCE，CLASS和RUNTIME？
I'm also not exactly sure what the phrase "retaining annotation" means.
我也不确定“保留注释”这个词是什么意思。
采纳答案by Favonius
RetentionPolicy.SOURCE: Discard during the compile. These annotations don't make any sense after the compile has completed, so they aren't written to the bytecode.
Example:@Override,@SuppressWarnings
RetentionPolicy.CLASS: Discard during class load. Useful when doing bytecode-level post-processing. Somewhat surprisingly, this is the default.
RetentionPolicy.RUNTIME: Do not discard. The annotation should be available for reflection at runtime. Example:@Deprecated
RetentionPolicy.SOURCE: 在编译过程中丢弃。编译完成后，这些注释没有任何意义，因此它们不会写入字节码。
示例：@Override,@SuppressWarnings
RetentionPolicy.CLASS: 在类加载期间丢弃。在进行字节码级别的后处理时很有用。有点令人惊讶的是，这是默认设置。
RetentionPolicy.RUNTIME： 不要丢弃。注释应该在运行时可用于反射。例子：@Deprecated
Source:The old URL is dead now
hunter_metaand replaced with hunter-meta-2-098036. In case even this goes down, I am uploading the image of the page.
来源：旧的 URL 现在已经失效了
hunter_meta并替换为hunter-meta-2-098036。万一出现这种情况，我正在上传页面的图像。
Image(Right Click and Select 'Open Image in New Tab/Window')
图像（右键单击并选择“在新选项卡/窗口中打开图像”）
回答by ewernli
According to your comments about class decompilation, here is how I think it should work:
根据您对类反编译的评论，我认为它应该如何工作：
RetentionPolicy.SOURCE: Won't appear in the decompiled classRetentionPolicy.CLASS: Appear in the decompiled class, but can't be inspected at run-time with reflection withgetAnnotations()RetentionPolicy.RUNTIME: Appear in the decompiled class, and can be inspected at run-time with reflection withgetAnnotations()
RetentionPolicy.SOURCE: 不会出现在反编译的类中RetentionPolicy.CLASS: 出现在反编译的类中，但不能在运行时用反射进行检查getAnnotations()RetentionPolicy.RUNTIME: 出现在反编译的类中，可以在运行时用反射进行检查getAnnotations()
回答by Ferdous Wahid
Retention Policy: A retention policy determines at what point an annotation is discarded. It is s specified using Java's built-in annotations: @Retention[About]
保留策略：保留策略确定在什么时候丢弃注释。它是使用 Java 的内置注释指定的：@Retention[关于]
1.SOURCE: annotation retained only in the source file and is discarded
during compilation.
2.CLASS: annotation stored in the .class file during compilation,
not available in the run time.
3.RUNTIME: annotation stored in the .class file and available in the run time.
回答by Michael Wong
- CLASS：Annotations are to be recorded in the class file by the compiler but need not be retained by the VM at run time.
- RUNTIME：Annotations are to be recorded in the class file by the compiler and retained by the VM at run time, so they may be read reflectively.
- SOURCE：Annotations are to be discarded by the compiler.
- CLASS：注解会被编译器记录在类文件中，但在运行时不需要被VM保留。
- RUNTIME：注解会被编译器记录在class文件中，并在运行时由VM保留，因此可以反射读取。
- SOURCE：注释将被编译器丢弃。
