ExpressionUtil

Introduction

Similar to template engines, Hutool encapsulates popular expression evaluation engines as a facade pattern, providing a unified API to eliminate differences. The existing engine implementations include:

• Aviator
• Apache Jexl3
• MVEL
• JfireEL
• Rhino
• Spring Expression Language (SpEL)

Usage

First, introduce the template engine we need. After the introduction, Hutool can automatically identify and use it through the SPI mechanism. Let’s take Aviator as an example:

<dependency>
 <groupId>com.googlecode.aviator</groupId>
 <artifactId>aviator</artifactId>
 <version>5.2.7</version>
</dependency>

Executing Expressions

final Dict dict = Dict.create()
 .set("a", 100.3)
 .set("b", 45)
 .set("c", -199.100);

// -143.8
final Object eval = ExpressionUtil.eval("a-(b-c)", dict);

Custom Engine Execution

If multiple engines are introduced in the project and we want to choose a specific engine to execute, we can do:

ExpressionEngine engine = new JexlEngine();

final Dict dict = Dict.create()
 .set("a", 100.3)
 .set("b", 45)
 .set("c", -199.100);

// -143.8
final Object eval = engine.eval("a-(b-c)", dict);

Creating a Custom Engine

The core of the engine is to implement the ExpressionEngine interface, which has only one method: eval. After implementing this interface, create a spi file cn.hutool.extra.expression.ExpressionEngine in the META-INF/services/ directory of the project:

com.yourProject.XXXXEngine

This way, you can directly call ExpressionUtil.eval to execute the expression.