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# ognl
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[`ognl`在线教程](https://arthas.aliyun.com/doc/arthas-tutorials?language=cn&id=command-ognl)
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::: tip
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执行 ognl 表达式
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:::
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从 3.0.5 版本增加
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## 参数说明
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| 参数名称 | 参数说明 |
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| --------------------: | :--------------------------------------------------------------- |
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| _express_ | 执行的表达式 |
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| `[c:]` | 执行表达式的 ClassLoader 的 hashcode，默认值是 SystemClassLoader |
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| `[classLoaderClass:]` | 指定执行表达式的 ClassLoader 的 class name |
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| [x] | 结果对象的展开层次，默认值 1 |
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## 使用参考
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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/dormant/commons-ognl/language-guide.html](https://commons.apache.org/dormant/commons-ognl/language-guide.html)
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调用静态函数：
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```bash
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$ ognl '@java.lang.System@out.println("hello")'
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null
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```
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获取静态类的静态字段：
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```bash
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$ ognl '@demo.MathGame@random'
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@Random[
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serialVersionUID=@Long[3905348978240129619],
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seed=@AtomicLong[125451474443703],
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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@28ea5898],
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seedOffset=@Long[24],
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]
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```
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通过 hashcode 指定 ClassLoader：
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```bash
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$ classloader -t
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+-BootstrapClassLoader
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+-jdk.internal.loader.ClassLoaders$PlatformClassLoader@301ec38b
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+-com.taobao.arthas.agent.ArthasClassloader@472067c7
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+-jdk.internal.loader.ClassLoaders$AppClassLoader@4b85612c
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+-org.springframework.boot.loader.LaunchedURLClassLoader@7f9a81e8
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$ ognl -c 7f9a81e8 @org.springframework.boot.SpringApplication@logger
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@Slf4jLocationAwareLog[
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FQCN=@String[org.apache.commons.logging.LogAdapter$Slf4jLocationAwareLog],
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name=@String[org.springframework.boot.SpringApplication],
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logger=@Logger[Logger[org.springframework.boot.SpringApplication]],
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]
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$
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```
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注意 hashcode 是变化的，需要先查看当前的 ClassLoader 信息，提取对应 ClassLoader 的 hashcode。
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对于只有唯一实例的 ClassLoader 可以通过 class name 指定，使用起来更加方便：
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```bash
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$ ognl --classLoaderClass org.springframework.boot.loader.LaunchedURLClassLoader @org.springframework.boot.SpringApplication@logger
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@Slf4jLocationAwareLog[
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FQCN=@String[org.apache.commons.logging.LogAdapter$Slf4jLocationAwareLog],
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name=@String[org.springframework.boot.SpringApplication],
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logger=@Logger[Logger[org.springframework.boot.SpringApplication]],
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]
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```
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执行多行表达式，赋值给临时变量，返回一个 List：
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```bash
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$ ognl '#value1=@System@getProperty("java.home"), #value2=@System@getProperty("java.runtime.name"), {#value1, #value2}'
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@ArrayList[
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@String[/opt/java/8.0.181-zulu/jre],
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@String[OpenJDK Runtime Environment],
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]
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```
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