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The working principle and application scenario of the JSP Standard Tag Library (JSTL) in the Java Library Overview: JSTL (JavaseerVer Pages Standard Tag Library) is a standard label library provided by Java to develop and manage common page logic and display in JSP pages.JSTL provides a set of labels and functions. Through these labels and functions, developers can separate business logic from the JSP page, making the page more concise, maintenance and reused. working principle: JSTL is based on the Java library.When running, the JSTL label will be parsed into the corresponding Java library code, and then executes it on the server.The labels and functions in the JSTL tag library provide some common functions, such as cycle, conditional judgment, database operations, etc.Developers can use these tags and functions on the JSP page to achieve complex business logic through simple configuration and use. Application scenario: 1. Circular and conditional judgment: The use of JSTL's cycle and conditional judgment labels can easily implement functions such as list display and condition branches.For example, you can use <C: Foreach> tags to traverse a collection, and display the information of each element on the page.For another example, you can use <c: if> and <c: choose> tags to implement conditional judgment and multi -branch options. <c:forEach items="${users}" var="user"> <tr> <td>${user.id}</td> <td>${user.name}</td> <td>${user.email}</td> </tr> </c:forEach> <c:choose> <c:when test="${salary < 5000}"> <p>The salary is low.</p> </c:when> <c:when test="${salary >= 5000}"> <p>The salary is moderate.</p> </c:when> <c:otherwise> <p>The salary is high.</p> </c:otherwise> </c:choose> 2. Database operation: The JSTL label library also provides labels for database operations.Developers can use these tags to connect databases, execute SQL queries, and display the results on the page.For example, you can use the <SQL: Setdatasource> tag configuration database connection, use the <sql: query> tag to perform query, and then use the <C: Foreach> tag circulation to display the results. <sql:setDataSource var="db" driver="com.mysql.jdbc.Driver" url="jdbc:mysql://localhost/test" user="root" password="password" /> <sql:query dataSource="${db}" var="result"> SELECT * FROM users WHERE age > 18 </sql:query> <c:forEach var="row" items="${result.rows}"> <tr> <td>${row.id}</td> <td>${row.name}</td> <td>${row.age}</td> </tr> </c:forEach> Summarize: JSTL is a powerful and flexible label library. Through it, developers can achieve complex business logic on the JSP page.Its working principle is to parse the label to the Java library code and execute it on the server.Using JSTL can make the page more concise, easy to read, and easy to maintain, improve development efficiency and code reuse.In the scenario, conditional judgment, and database operations, JSTL is a very useful and convenient tool.