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getstatic
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===
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[`getstatic`在线教程](https://arthas.aliyun.com/doc/arthas-tutorials.html?language=cn&id=command-getstatic)
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* 推荐直接使用[ognl](ognl.md)命令，更加灵活。
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通过getstatic命令可以方便的查看类的静态属性。使用方法为`getstatic class_name field_name`
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```bash
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$ getstatic demo.MathGame random
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field: random
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@Random[
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serialVersionUID=@Long[3905348978240129619],
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seed=@AtomicLong[120955813885284],
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multiplier=@Long[25214903917],
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addend=@Long[11],
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mask=@Long[281474976710655],
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DOUBLE_UNIT=@Double[1.1102230246251565E-16],
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BadBound=@String[bound must be positive],
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BadRange=@String[bound must be greater than origin],
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BadSize=@String[size must be non-negative],
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seedUniquifier=@AtomicLong[-3282039941672302964],
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nextNextGaussian=@Double[0.0],
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haveNextNextGaussian=@Boolean[false],
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serialPersistentFields=@ObjectStreamField[][isEmpty=false;size=3],
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unsafe=@Unsafe[sun.misc.Unsafe@2eaa1027],
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seedOffset=@Long[24],
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]
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```
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* 指定classLoader
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注意hashcode是变化的，需要先查看当前的ClassLoader信息，使用`sc -d <ClassName>`提取对应ClassLoader的hashcode。
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如果你使用`-c`，你需要手动输入hashcode：`-c <hashcode>`
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```bash
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$ getstatic -c 3d4eac69 demo.MathGame random
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```
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对于只有唯一实例的ClassLoader可以通过`--classLoaderClass`指定class name，使用起来更加方便：
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`getstatic --classLoaderClass sun.misc.Launcher$AppClassLoader demo.MathGame random`
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* 注: 这里classLoaderClass 在 java 8 是 sun.misc.Launcher$AppClassLoader，而java 11的classloader是jdk.internal.loader.ClassLoaders$AppClassLoader，katacoda目前环境是java8。
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`--classLoaderClass` 的值是ClassLoader的类名，只有匹配到唯一的ClassLoader实例时才能工作，目的是方便输入通用命令，而`-c <hashcode>`是动态变化的。
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如果该静态属性是一个复杂对象，还可以支持在该属性上通过ognl表示进行遍历，过滤，访问对象的内部属性等操作。
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* OGNL特殊用法请参考：[https://github.com/alibaba/arthas/issues/71](https://github.com/alibaba/arthas/issues/71)
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* OGNL表达式官方指南：[https://commons.apache.org/proper/commons-ognl/language-guide.html](https://commons.apache.org/proper/commons-ognl/language-guide.html)
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例如，假设n是一个Map，Map的Key是一个Enum，我们想过滤出Map中Key为某个Enum的值，可以写如下命令
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```
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$ getstatic com.alibaba.arthas.Test n 'entrySet().iterator.{? #this.key.name()=="STOP"}'
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field: n
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@ArrayList[
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@Node[STOP=bbb],
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]
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Affect(row-cnt:1) cost in 68 ms.
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$ getstatic com.alibaba.arthas.Test m 'entrySet().iterator.{? #this.key=="a"}'
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field: m
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@ArrayList[
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@Node[a=aaa],
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]
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```
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