Use the Jakarta expression language API to build a Java class library: the best practice guide
Use the Jakarta expression language API to build a Java class library: the best practice guide
introduce:
Jakarta expression language (Jexl) is a simplified expression language based on Java language, which allows Java developers to evaluate and execute dynamic expressions during runtime.Jexl provides a simple and powerful way to operate and retrieve the attributes and methods of Java objects through expressions.
This article will introduce how to use the Jakarta expression language API to build a Java class library and provide some best practical guidelines to help you better apply this powerful tool.
Step 1: Add dependencies
First, add the Jexl API to your project.You can achieve this by adding the following dependencies in your construction tools (such as Maven or Gradle):
<dependency>
<groupId>org.apache.commons</groupId>
<artifactId>commons-jexl3</artifactId>
<version>3.2</version>
</dependency>
Step 2: Create a JEXL engine
Before using Jexl, we need to create a Jexl engine.The JEXL engine is responsible for handling and analyzing expressions and performing corresponding operations.You can create a Jexl engine through the following code:
import org.apache.commons.jexl3.JexlEngine;
JexlEngine jexlEngine = new JexlEngine();
Step 3: Define the expression
Next, you need to define an expression, which is achieved through a string.The expression can contain variables, function calls, attribute access, and so on.For example, the following is a simple expression:
String expression = "name.length() > 5";
Step 4: Compile expression
Once we define expressions, we need to compile them into executable code.This can be implemented by using the Jexl engine's `CreateExpression () method:
import org.apache.commons.jexl3.Expression;
Expression expr = jexlEngine.createExpression(expression);
Step 5: Express expression
Now, we are ready to execute the expression.To perform expressions, we need to create a Jexl context and store the value of the variable in the context.Then, we can use the context's `Evaluate ()` method to calculate and return the results of the expression:
import org.apache.commons.jexl3.JexlContext;
import org.apache.commons.jexl3.MapContext;
JexlContext context = new MapContext();
context.set("name", "John");
Boolean result = (Boolean) expr.evaluate(context);
In the above example, we pass the variables called "John" to the expression and calculate the results of the expression.In this case, the expression will return a Boolean value to indicate whether the length of the name is greater than 5.
Step 6: Process function call and attribute access
Jexl also allows you to expand the expression in a way to call and attribute access.You can add an instance of the Java class to the context and call it or access its attributes in the expression.
The following is an example that shows how to add a Person object to the context, and call its getname () method in the expression:
public class Person {
private String name;
public String getName() {
return name;
}
}
Person person = new Person();
person.setName("Alice");
context.set("person", person);
String expression = "person.getName()";
Expression expr = jexlEngine.createExpression(expression);
String result = (String) expr.evaluate(context);
In this example, the expression will return "Alice", the name of the Person object.
Best Practices:
-Rested to reuse the Jexl engine: The creation of the Jexl engine is relatively expensive, so if possible, try to reuse the same engine as much as possible.
-Re reduce the number of expression compilation: If possible, avoid re -compilation every time the expression is executed.A common method is to cache the compiled expression for reuse.
-Dutoning expression: Jexl provides some debugging functions to help you exclude problems when performing expressions.You can turn on the debug mode of the Jexl engine and get the compiled expression source code when needed to debug.
Summarize:
This article introduces how to use the JAKARTA expression language API to build the best practice of the Java library.By using the Jexl engine, defining expressions, compilation and execution expressions, you can use Jexl's powerful functions to perform dynamic attributes and methods.Following the best practice provided here, you will be able to better use the Jakarta expression language API to build a flexible and efficient Java class library.
