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## FAQ
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> 不在本列表里的问题，请到issue里搜索。 [https://github.com/alibaba/arthas/issues](https://github.com/alibaba/arthas/issues)
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##### Arthas attach之后对原进程性能有多大的影响
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[https://github.com/alibaba/arthas/issues/44](https://github.com/alibaba/arthas/issues/44)
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##### trace/watch等命令能否增强jdk里的类？
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默认情况下会过滤掉`java.`开头的类，但可以通过参数开启。
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```bash
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options unsafe true
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```
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更多参考 [options](options.md)
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##### 怎么以`json`格式查看结果
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```bash
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options json-format true
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```
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更多参考 [options](options.md)
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##### Arthas能否跟踪 native 函数
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不能。
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##### 能不能查看内存里某个变量的值
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不能。但可以用一些技巧，用`tt`命令拦截到对象，或者从静态函数里取到对象。
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##### 方法同名过滤
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同名方法过滤可以通过匹配表达式,可以使用[表达式核心变量](advice-class.md)中所有变量作为已知条件,可以通过判断参数个数`params.length ==1`, 参数类型`params[0] instanceof java.lang.Integer`、返回值类型 `returnObj instanceof java.util.List` 等等一种或者多种组合进行过滤。
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可以使用 `-v` 查看观察匹配表达式的执行结果 [https://github.com/alibaba/arthas/issues/1348](https://github.com/alibaba/arthas/issues/1348)
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例子[arthas-demo](quick-start.md)
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```bash
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watch demo.MathGame primeFactors traceE '{params,returnObj,throwExp}' -v -n 5 -x 3 'params.length >0 && returnObj instanceof java.util.List'
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```
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##### 怎么watch、trace 构造函数 ？
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```bash
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watch demo.MathGame <init> '{params,returnObj,throwExp}' -v -n 5 -x 3 '1==1'
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```
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##### java.lang.ClassFormatError: null、skywalking arthas 兼容使用
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当出现这个错误日志`java.lang.ClassFormatError: null`,通常情况下都是被其他字节码工具修改过与arthas修改字节码不兼容。
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比如: 使用 skywalking V8.1.0 以下版本 [无法trace、watch 被skywalking agent 增强过的类](https://github.com/alibaba/arthas/issues/1141), V8.1.0 以上版本可以兼容使用,更多参考skywalking配置 [skywalking compatible with other javaagent bytecode processing](https://github.com/apache/skywalking/blob/v8.1.0/docs/en/FAQ/Compatible-with-other-javaagent-bytecode-processing.md)。
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