How to call javascript in java

When developing web applications, sometimes you need to call JavaScript code in the Java back-end code. In this case, Java's scripting engine can be used to execute JavaScript code. This article will introduce how to call JavaScript in Java.

1. Introducing the script engine library

There are many script engines available in Java, such as Rhino and Nashorn. In this article, we will use Nashorn as an example.

To use Nashorn in Java, you need to introduce the Nashorn library into the project. If you use Maven to build, just add the following dependencies in the pom.xml file:

<dependency>
   <groupid>org.openjdk.nashorn</groupid>
   <artifactid>nashorn-core</artifactid>
   <version>1.8.0_25</version>
</dependency>

2. Writing JavaScript code

Before calling JavaScript code in Java, you need to Write JavaScript code. Here is a simple JavaScript example:

var calculate = function(a, b) {
   return a + b;
}

This code defines a function called calculate that adds two arguments and returns the result.

3. Calling JavaScript code in Java

There are many ways to call JavaScript code in Java, such as loading JavaScript files through script engines, executing JavaScript strings, etc. The following is some sample code:

3.1. Loading JavaScript files through the script engine

In Java, JavaScript files can be loaded through the script engine and the code in them is executed. The following is a sample code:

ScriptEngineManager manager = new ScriptEngineManager();
ScriptEngine engine = manager.getEngineByName("JavaScript");
engine.eval(new FileReader("path/to/javascript/file.js"));
Invocable invocable = (Invocable) engine;
Object result = invocable.invokeFunction("calculate", 1, 2);
System.out.println(result);

In this code, first use ScriptEngineManager to create a JavaScript script engine. Then load the JavaScript file and execute the code in it by calling the eval method. Next, create an Invocable object and call its invokeFunction method to call the JavaScript function. Finally, print the result of the function.

3.2. Execute JavaScript strings

In addition to calling JavaScript code by loading files, you can also call code by executing JavaScript strings. The following is a sample code:

String script = "var calculate = function(a, b) {  return a + b; }";
ScriptEngineManager manager = new ScriptEngineManager();
ScriptEngine engine = manager.getEngineByName("JavaScript");
engine.eval(script);
Invocable invocable = (Invocable) engine;
Object result = invocable.invokeFunction("calculate", 1, 2);
System.out.println(result);

In this code, a JavaScript string is first defined, which contains the definition of a function named calculate. Then, create a JavaScript script engine and call its eval method to execute this string. Finally, call the invokeFunction method again to call the function.

4. Summary

This article introduces how to call JavaScript code in Java. JavaScript code can be easily executed from Java code by using Java's Nashorn scripting engine. With this capability, web developers can more easily integrate front-end and back-end logic into the same application.

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