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JSP cannot run JavaScript

王林

May 09, 2023 pm 10:31 PM

In recent years, with the rapid development of the Internet, JSP plays an increasingly important role in Java Web development. As one of the core technologies of Web front-end development, JavaScript is also increasingly widely used. However, in actual development, we often encounter situations where JSP cannot run JavaScript. So, what is the reason for this problem? How to solve? This article will conduct analysis in the following aspects.

1. The order problem of JavaScript execution and page rendering

The order problem of JavaScript execution and page rendering is a common problem that JSP cannot run JavaScript. In a JSP page, when the browser parses the JavaScript script, the JavaScript code block has not been fully loaded, so when the JavaScript script on the JSP page is executed, the page may not have been fully rendered. This may cause the JavaScript script to error or fail to run. For example, the following code example:

<html>
<head>
    <meta charset="UTF-8">
    <title>JavaScript无法运行的示例</title>
</head>
<body>
    <div id="demo"></div>
    <script>
        document.getElementById("demo").innerHTML = "Hello,JavaScript!";
    </script>
</body>
</html>

In the above code example, we try to modify the innerHTML content of a div with the id of demo through JavaScript and output the text Hello, JavaScript!. However, executing the JavaScript script before the page is rendered will cause an error because the div with the ID demo has not yet been rendered. At this time, the operation of modifying innerHTML will fail.

In order to solve this problem, we can put the JavaScript code to be executed after the page is loaded, that is, put the code to be executed inside the window.onload event handling function. In this way, the above problems will not occur when the JavaScript script is executed after the page is loaded. The modified code example is as follows:

<html>
<head>
    <meta charset="UTF-8">
    <title>JavaScript能够运行的示例</title>
</head>
<body>
    <div id="demo"></div>
    <script>
        window.onload = function() {
            document.getElementById("demo").innerHTML = "Hello,JavaScript!";
        }
    </script>
</body>
</html>

2. The problem of referencing JavaScript code in JSP pages

In JSP pages, if JavaScript files or scripts are not referenced correctly, JavaScript may not be able to run. The problem. As shown in the following code example:

<html>
<head>
    <meta charset="UTF-8">
    <title>JavaScript无法运行的示例</title>
</head>
<body>
    <div id="demo"></div>
    <script src="js/demo.js"></script>
    <script>
        document.getElementById("demo").innerHTML = "Hello,JavaScript!";
    </script>
</body>
</html>

In the above code example, we referenced a JavaScript file demo.js, but if the file does not exist or the path is incorrect, JavaScript cannot run. In actual development, to avoid such problems, we should carefully check whether the path and name of the JavaScript file are correct.

3. Problems with the interaction between JSP pages and JavaScript

When JSP pages interact with JavaScript, JavaScript may not run. For example, in a JSP page, we can use EL expressions in JSP (such as ${}) to obtain background Java data, and then pass these data to the JavaScript script. However, if the data format obtained in the El expression is incorrect, or the data is not passed correctly, JavaScript will not run. Like the following code example:

<html>
<head>
    <meta charset="UTF-8">
    <title>JavaScript无法运行的示例</title>
</head>
<body>
    <div id="demo"></div>
    <%
        String name = "张三";
    %>
    <script>
        var name = "${name}";
        document.getElementById("demo").innerHTML = "Hello," + name + "!";
    </script>
</body>
</html>

In the above code example, we get a string named "name" and then pass it to the JavaScript script. However, because the value of name is not enclosed in double quotes, this JavaScript code will report an error because the value of name is not a valid string.

In order to avoid this problem, we can pass the data correctly in the EL expression, and check the passed data in the JavaScript code to ensure that the data format is correct. The modified code example is as follows:

<html>
<head>
    <meta charset="UTF-8">
    <title>JavaScript能够运行的示例</title>
</head>
<body>
    <div id="demo"></div>
    <%
        String name = "张三";
    %>
    <script>
        var name = "${name}";
        if (typeof name != 'undefined' && name != null) {
            document.getElementById("demo").innerHTML = "Hello," + name + "!";
        }
    </script>
</body>
</html>

The above is a detailed analysis of the reasons and solutions for JSP's inability to run JavaScript. We need to pay attention to these issues in actual development to avoid errors caused by the inability to run JavaScript. Looking to the future, with the continuous development of JSP and JavaScript technology, we believe that these problems will be solved with the advancement of technology and provide better support for web development.

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