如何从JavaScript调用Java类方法？附Applet应用场景

Hey there! Since you already have your edu.MGT.MainApplet running via the <applet> tag, connecting JavaScript to its methods is totally doable using LiveConnect—the technology that enables communication between JavaScript and Java in browsers. Let's walk through the steps:

1. Update Your Applet Tag with an Identifier #

First, make sure your <applet> tag has an id (or name) attribute so JavaScript can easily reference the applet instance. If you don't need a visible UI for the applet, you can set width and height to 0 to hide it:

<applet 
    code="edu.MGT.MainApplet.class" 
    archive="MGT2-0.0.1.jar"
    id="mathToolApplet"
    width="0" 
    height="0"
>
    Your browser does not support Java Applets.
</applet>

2. Access the Applet Instance in JavaScript #

Once the applet is loaded, you can grab its reference using document.getElementById():

// Get the applet instance
const mathApplet = document.getElementById('mathToolApplet');

3. Call Public Methods Directly #

JavaScript can invoke public methods on the applet instance. Just make sure the methods you want to call are marked public in your Java code, and pay attention to parameter type matching (e.g., Java int ↔ JS number, Java String ↔ JS string).

Example Calls (Matching Your Math Expression Tool) #

Let's use hypothetical methods from your applet to demonstrate:

// Call a method to evaluate a math expression
const evalResult = mathApplet.evaluateExpression('3 * (4 + sin(pi/2))');
console.log('Evaluation Result:', evalResult);

// Call a method to compute a derivative
const derivative = mathApplet.computeDerivative('x^2 + 5x');
console.log('Derivative:', derivative);

// Access a returned data tree object (and its public methods/properties)
const exprTree = mathApplet.parseExpression('log(x) + exp(2x)');
// Assuming your tree class has a public toString() method
console.log('Expression Tree:', exprTree.toString());

Critical Notes to Avoid Issues #

• Wait for the Applet to Load: Don't call methods before the applet is fully initialized. Use the onload event to trigger your JavaScript logic:

  <applet 
    <!-- ... other attributes ... -->
    onload="onMathAppletReady()"
  ></applet>
  
  <script>
  function onMathAppletReady() {
      console.log('Applet is ready to use!');
      // Start calling methods here
  }
  </script>
  

• Security Restrictions: Unsigned applets have limited permissions (e.g., can't access local files, may be blocked from cross-origin interactions). If you hit permission errors, you'll need to sign your MGT2-0.0.1.jar with a code signing certificate.
• Browser Support: Keep in mind that modern browsers (Chrome, Firefox, Edge) have dropped support for Java Applets. This approach works best in older browsers or environments where Java plugin support is enabled (like specialized enterprise browsers).
• Error Handling: Wrap calls in try/catch to handle cases where methods fail or the applet isn't available:

  try {
      const result = mathApplet.findSubexpression('x^2 + 5x', 'x');
      console.log('Found Subexpression:', result);
  } catch (error) {
      console.error('Failed to call applet method:', error);
  }
  

内容的提问来源于stack exchange，提问作者pasaba por aqui