# Fundamental Fields in Expressions

There is a very fundamental class `Advice` for the expressions used in filtering, tracing or monitoring and other aspects in commands.

```java
public class Advice {

    private final ClassLoader loader;
    private final Class<?> clazz;
    private final ArthasMethod method;
    private final Object target;
    private final Object[] params;
    private final Object returnObj;
    private final Throwable throwExp;
    private final boolean isBefore;
    private final boolean isThrow;
    private final boolean isReturn;

    // getter/setter
}
```

Description for the variables in the class `Advice`:

|      Name | Specification                                                                                                                                                       |
| --------: | :------------------------------------------------------------------------------------------------------------------------------------------------------------------ |
|    loader | the class loader for the current called class                                                                                                                       |
|     clazz | the reference to the current called class                                                                                                                           |
|    method | the reference to the current called method                                                                                                                          |
|    target | the instance of the current called class                                                                                                                            |
|    params | the parameters for the current call, which is an array (when there's no parameter, it will be an empty array)                                                       |
| returnObj | the return value from the current call - only available when the method call returns normally (`isReturn==true`), and `null` is for `void` return value             |
|  throwExp | the exceptions thrown from the current call - only available when the method call throws exception (`isThrow==true`)                                                |
|  isBefore | flag to indicate the method is about to execute. `isBefore==true` but `isThrow==false` and `isReturn==false` since it's no way to know how the method call will end |
|   isThrow | flag to indicate the method call ends with exception thrown                                                                                                         |
|  isReturn | flag to indicate the method call ends normally without exception thrown                                                                                             |

All variables listed above can be used directly in the [OGNL expression](https://commons.apache.org/proper/commons-ognl/language-guide.html). The command will not execute and exit if there's illegal OGNL grammar or unexpected variable in the expression.

- [typical use cases](https://github.com/alibaba/arthas/issues/71);
- [OGNL language guide](https://commons.apache.org/proper/commons-ognl/language-guide.html).