给用户随机推荐不是自己发布的笔记，报错了。

Question

给用户随机推荐不是自己发布的笔记，但是不知道哪里错了。代码如下：

    private List<Notebook> getRandomNotebook(int num) {
        List<Notebook> list = new ArrayList<>(num);

        List<Notebook> notebook = notebookMapper.selectNotebookItem(null);
        for (int i = 0; i < num; i++) {
            int index = new Random().nextInt(notebook.size());
            list.add(notebook.get(index));
        }
        return list;
    }

 List<Notebook> selectNotebookItem(Notebook notebook);

        <select id="selectNotebookItem" resultType="com.example.entity.Notebook">
            SELECT
            notebook.*,
            user.username AS userName,
            user.avatar AS avatar
            FROM
            notebook
            LEFT JOIN
            user
            ON
            notebook.user_id = user.id
            WHERE
            user_id != #{userId}
        </select>

报错如下：

2024-05-23 10:43:34.694 ERROR 31548 --- [nio-9090-exec-6] c.e.exception.GlobalExceptionHandler     : 异常信息：

java.lang.IllegalArgumentException: bound must be positive
    at java.util.Random.nextInt(Random.java:388) ~[na:1.8.0_351]
    at com.example.service.NotebookService.getRandomNotebook(NotebookService.java:241) ~[classes/:na]
    at com.example.service.NotebookService.recommendNotebook(NotebookService.java:219) ~[classes/:na]
    at com.example.controller.NotebookController.recommendNotebook(NotebookController.java:111) ~[classes/:na]
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_351]
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_351]
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_351]
    at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_351]
    at org.springframework.web.method.support.InvocableHandlerMethod.doInvoke(InvocableHandlerMethod.java:205) ~[spring-web-5.3.15.jar:5.3.15]
    at org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:150) ~[spring-web-5.3.15.jar:5.3.15]
    at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:117) ~[spring-webmvc-5.3.15.jar:5.3.15]
    at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:895) ~[spring-webmvc-5.3.15.jar:5.3.15]
    at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:808) ~[spring-webmvc-5.3.15.jar:5.3.15]
    at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:87) ~[spring-webmvc-5.3.15.jar:5.3.15]
    at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:1067) ~[spring-webmvc-5.3.15.jar:5.3.15]
    at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:963) ~[spring-webmvc-5.3.15.jar:5.3.15]
    at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:1006) [spring-webmvc-5.3.15.jar:5.3.15]
    at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:898) [spring-webmvc-5.3.15.jar:5.3.15]
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:655) [tomcat-embed-core-9.0.56.jar:4.0.FR]
    at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:883) [spring-webmvc-5.3.15.jar:5.3.15]
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:764) [tomcat-embed-core-9.0.56.jar:4.0.FR]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:227) [tomcat-embed-core-9.0.56.jar:9.0.56]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:162) [tomcat-embed-core-9.0.56.jar:9.0.56]
    at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53) [tomcat-embed-websocket-9.0.56.jar:9.0.56]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:189) [tomcat-embed-core-9.0.56.jar:9.0.56]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:162) [tomcat-embed-core-9.0.56.jar:9.0.56]
    at org.springframework.web.filter.CorsFilter.doFilterInternal(CorsFilter.java:91) [spring-web-5.3.15.jar:5.3.15]
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:117) [spring-web-5.3.15.jar:5.3.15]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:189) [tomcat-embed-core-9.0.56.jar:9.0.56]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:162) [tomcat-embed-core-9.0.56.jar:9.0.56]
    at org.springframework.web.filter.RequestContextFilter.doFilterInternal(RequestContextFilter.java:100) [spring-web-5.3.15.jar:5.3.15]
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:117) [spring-web-5.3.15.jar:5.3.15]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:189) [tomcat-embed-core-9.0.56.jar:9.0.56]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:162) [tomcat-embed-core-9.0.56.jar:9.0.56]
    at org.springframework.web.filter.FormContentFilter.doFilterInternal(FormContentFilter.java:93) [spring-web-5.3.15.jar:5.3.15]
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:117) [spring-web-5.3.15.jar:5.3.15]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:189) [tomcat-embed-core-9.0.56.jar:9.0.56]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:162) [tomcat-embed-core-9.0.56.jar:9.0.56]
    at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:201) [spring-web-5.3.15.jar:5.3.15]
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:117) [spring-web-5.3.15.jar:5.3.15]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:189) [tomcat-embed-core-9.0.56.jar:9.0.56]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:162) [tomcat-embed-core-9.0.56.jar:9.0.56]
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:197) [tomcat-embed-core-9.0.56.jar:9.0.56]
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:97) [tomcat-embed-core-9.0.56.jar:9.0.56]
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:540) [tomcat-embed-core-9.0.56.jar:9.0.56]
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:135) [tomcat-embed-core-9.0.56.jar:9.0.56]
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:92) [tomcat-embed-core-9.0.56.jar:9.0.56]
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:78) [tomcat-embed-core-9.0.56.jar:9.0.56]
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:357) [tomcat-embed-core-9.0.56.jar:9.0.56]
    at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:382) [tomcat-embed-core-9.0.56.jar:9.0.56]
    at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:65) [tomcat-embed-core-9.0.56.jar:9.0.56]
    at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:895) [tomcat-embed-core-9.0.56.jar:9.0.56]
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1732) [tomcat-embed-core-9.0.56.jar:9.0.56]
    at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49) [tomcat-embed-core-9.0.56.jar:9.0.56]
    at org.apache.tomcat.util.threads.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1191) [tomcat-embed-core-9.0.56.jar:9.0.56]
    at org.apache.tomcat.util.threads.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:659) [tomcat-embed-core-9.0.56.jar:9.0.56]
    at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) [tomcat-embed-core-9.0.56.jar:9.0.56]
    at java.lang.Thread.run(Thread.java:750) [na:1.8.0_351]

Answer 1

让阿豪来帮你解答，本回答参考chatgpt3.5编写提供，如果还有疑问可以评论或留言

问题描述： 代码中提供了一个方法，用于获取随机的不是当前用户发布的笔记。但是在运行时出现了错误，错误信息为：java.lang.IllegalArgumentException: bound must be positive。 问题原因： 错误信息中提示 bound must be positive，说明错误出现在 Random() 的 nextInt() 方法上。经过分析，我们可以发现方法中有以下代码： int index = new Random().nextInt(notebook.size()); 在 Random() 的构造方法中是没有指定 seed 的，所以每次调用 Random() 都会产生一个新的随机数生成器。这样会导致每次执行方法时都生成新的随机数，即使传入相同的参数也无法保证结果一致。 继续分析错误信息，可以发现错误出现在 nextInt() 方法上。nextInt() 方法的参数是范围的上界，如果上界为负数或者为 0，则会抛出 IllegalArgumentException 异常。 因此，问题的出现是因为随机范围的上界为 0，即 notebook.size() 的值为 0。这个值为 0 的原因可能是数据库中没有符合条件的记录，或者是查询条件有误。 解决方案： 对于随机范围的上界为 0 的情况，我们需要在方法开始前做一些判断，如果获取到的结果集为空，则直接返回一个空的 List。代码示例如下： private List getRandomNotebook(int num) { List list = new ArrayList<>(num); List notebook = notebookMapper.selectNotebookItem(null); if (notebook == null || notebook.isEmpty()) { return list; } for (int i = 0; i < num; i++) { int index = new Random().nextInt(notebook.size()); list.add(notebook.get(index)); } return list; } 注意事项： 1. 在使用 Random() 时，如果需要多次生成随机数，应该使用同一个随机数生成器，而不是每次都创建一个新的。 2. 获取随机数时，需要保证参数大于 0，否则会抛出 IllegalArgumentException 异常。