java - Spring 表单验证错误 : java. lang.IllegalArgumentException: 'items' must not be null-6ren

java - Spring 表单验证错误 : java. lang.IllegalArgumentException: 'items' must not be null

转载作者：搜寻专家更新时间：2023-11-01 03:05:27

我正在尝试 Validate Spring 表单，其中包括:

普通输入框
在 Controller 中设置选项的单选按钮(稍后可能来自数据库)
复选框，其选项也在 Controller 中设置
选择同样在 Controller 中设置的字段

Method 1 approach is followed.

型号:

public class Student {
    private Long id;

    @NotEmpty
    private String name;

    @NotEmpty
    private String address;

    @NotEmpty
    private String contactNumber;

    @NotEmpty
    private String grade;

    @NotEmpty
    private String gender;

    @NotEmpty
    @Email
    private String email;

    @NotEmpty
    private List<String> extraSubjects = new ArrayList<String>();

    /****  Corresponding getter setters ****/

}

Controller :-

@Controller
@RequestMapping("/student")
public class StudentController {
    private Logger logger = Logger.getLogger(StudentController.class);

    @RequestMapping("/home")
    public String getHomePage() {
        return "home";
    }

    @RequestMapping(value = "/add", method = RequestMethod.GET)
    public String addStudentForm(Model model) {
        Student student = new Student();
        model.addAttribute("studentForm", student);

        LinkedHashMap<Integer, String> gradeList = new LinkedHashMap<Integer, String>();
        gradeList.put(1, "Class 1");
        gradeList.put(2, "Class 2");
        gradeList.put(3, "Class 3");
        gradeList.put(4, "Class 4");
        gradeList.put(5, "Class 5");
        model.addAttribute("gradeList", gradeList);

        List<String> genderList = new ArrayList<String>();
        genderList.add("Male");
        genderList.add("Female");
        model.addAttribute("genderList", genderList);
        student.setGender("Male");

        List<String> extraSubjects = new ArrayList<String>();
        extraSubjects.add("Math II");
        extraSubjects.add("Population");

        List<String> extraSubjectsStd = new ArrayList<String>();
        extraSubjectsStd.add("Math II");
        extraSubjectsStd.add("Population");
        student.setExtraSubjects(extraSubjectsStd);

        extraSubjects.add("Computer");
        model.addAttribute("extraSubjects", extraSubjects);
        return "addstudent";
    }

    @RequestMapping(value = "/add", method = RequestMethod.POST)
    public String addStudent(
            @Valid @ModelAttribute("studentForm") Student student,
            BindingResult result) {

        if (result.hasErrors()) {
            logger.info("Returning add page with errors");
            return "addstudent";
        }
            //Stuffs to save in database 
        return "redirect:add";
    }
}

JSP 页面:-

<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8"%>
<%@ taglib uri="http://www.springframework.org/tags" prefix="spring"%>
<%@ taglib uri="http://www.springframework.org/tags/form" prefix="form"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>Insert title here</title>
<style>
.error {
    color: #ff0000;
}

.errorblock {
    color: #000;
    background-color: #ffEEEE;
    border: 3px solid #ff0000;
    padding: 8px;
    margin: 16px;
}
</style>
</head>
<body>
    <form:form action="add" method="post" commandName="studentForm">
        <form:errors path="*" cssClass="errorblock" element="div" />
        <table border="0">
            <tr>
                <td colspan="3" align="center"><h3>Add Student</h3></td>
            </tr>
            <tr>
                <td><form:label path="name">
                        <spring:message code="label.studentname" />
                    </form:label></td>
                <td><form:input path="name" /></td>
                <td><form:errors path="name" cssClass="error" /></td>
            </tr>
            <tr>
                <td><form:label path="address">
                        <spring:message code="label.address" />
                    </form:label></td>
                <td><form:input path="address" /></td>
                <td><form:errors path="address" cssClass="error" />
            </tr>
            <tr>
                <td>Contact Number:</td>
                <td><form:input path="contactNumber" /></td>
                <td><form:errors path="contactNumber" cssClass="error" />
            </tr>
            <tr>
                <td><form:label path="grade">
                        <spring:message code="label.grade" />
                    </form:label></td>
                <td><form:select path="grade">
                        <form:option value="" label="--- Select ---" />
                        <form:options items="${gradeList}" />
                    </form:select></td>
                <td><form:errors path="grade" cssClass="error" />
            </tr>
            <tr>
                <td>Gender:</td>
                <td><form:radiobuttons path="gender" items="${genderList}" />
                </td>
                <td><form:errors path="gender" cssClass="error" />
            </tr>
            <tr>
                <td>Extra Subjects:</td>
                <td><form:checkboxes items="${extraSubjects}"
                        path="extraSubjects" /></td>
                <td><form:errors path="extraSubjects" cssClass="error" />
            </tr>
            <tr>
                <td>Email Address:</td>
                <td><form:input items="${email}" path="email" /></td>
                <td><form:errors path="email" cssClass="error" />
            </tr>
            <tr>
                <td></td>
                <td align="center"><input type="submit" value="Save" /></td>
                <td></td>
            </tr>
        </table>
    </form:form>
</body>
</html>

现在，当我运行我的应用程序时，表单会在我执行获取操作 时加载。 单选按钮、选择字段和复选框中的值也被加载并被选中。如果我提交没有验证错误的表单，它工作正常但是当我发送有验证错误的表单时，我会收到以下错误:

堆栈跟踪:-

Jun 26, 2014 4:32:00 PM org.apache.catalina.core.ApplicationDispatcher invoke
SEVERE: Servlet.service() for servlet jsp threw exception
java.lang.IllegalArgumentException: 'items' must not be null
    at org.springframework.util.Assert.notNull(Assert.java:112)
    at org.springframework.web.servlet.tags.form.AbstractMultiCheckedElementTag.setItems(AbstractMultiCheckedElementTag.java:85)
    at org.apache.jsp.WEB_002dINF.pages.addstudent_jsp._jspx_meth_form_005fradiobuttons_005f0(addstudent_jsp.java:810)
    at org.apache.jsp.WEB_002dINF.pages.addstudent_jsp._jspService(addstudent_jsp.java:360)
    at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:727)
    at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:432)
    at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:390)
    at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:334)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:727)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:303)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
    at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
    at org.apache.catalina.core.ApplicationDispatcher.invoke(ApplicationDispatcher.java:748)
    at org.apache.catalina.core.ApplicationDispatcher.processRequest(ApplicationDispatcher.java:486)
    at org.apache.catalina.core.ApplicationDispatcher.doForward(ApplicationDispatcher.java:411)
    at org.apache.catalina.core.ApplicationDispatcher.forward(ApplicationDispatcher.java:338)
    at org.springframework.web.servlet.view.InternalResourceView.renderMergedOutputModel(InternalResourceView.java:209)
    at org.springframework.web.servlet.view.AbstractView.render(AbstractView.java:266)
    at org.springframework.web.servlet.DispatcherServlet.render(DispatcherServlet.java:1225)
    at org.springframework.web.servlet.DispatcherServlet.processDispatchResult(DispatcherServlet.java:1012)
    at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:959)
    at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:876)
    at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:931)
    at org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:833)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:646)
    at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:807)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:727)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:303)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
    at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:220)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:122)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:501)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:170)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:98)
    at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:950)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:116)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:408)
    at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1040)
    at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:607)
    at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:313)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
    at java.lang.Thread.run(Unknown Source)

DEBUG org.springframework.web.servlet.DispatcherServlet- Error rendering view [org.springframework.web.servlet.view.JstlView: name 'addstudent'; URL [/WEB-INF/pages/addstudent.jsp]] in DispatcherServlet with name 'mvc-dispatcher'
org.apache.jasper.JasperException: An exception occurred processing JSP page /WEB-INF/pages/addstudent.jsp at line 62

59:             </tr>
60:             <tr>
61:                 <td>Gender:</td>
62:                 <td><form:radiobuttons path="gender" items="${genderList}" />
63:                 </td>
64:                 <td><form:errors path="gender" cssClass="error" />
65:             </tr>    

Stacktrace:
        at org.apache.jasper.servlet.JspServletWrapper.handleJspException(JspServletWrapper.java:568)
        at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:470)
        at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:390)
        at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:334)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:727)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:303)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
        at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
        at org.apache.catalina.core.ApplicationDispatcher.invoke(ApplicationDispatcher.java:748)
        at org.apache.catalina.core.ApplicationDispatcher.processRequest(ApplicationDispatcher.java:486)
        at org.apache.catalina.core.ApplicationDispatcher.doForward(ApplicationDispatcher.java:411)
        at org.apache.catalina.core.ApplicationDispatcher.forward(ApplicationDispatcher.java:338)
        at org.springframework.web.servlet.view.InternalResourceView.renderMergedOutputModel(InternalResourceView.java:209)
        at org.springframework.web.servlet.view.AbstractView.render(AbstractView.java:266)
        at org.springframework.web.servlet.DispatcherServlet.render(DispatcherServlet.java:1225)
        at org.springframework.web.servlet.DispatcherServlet.processDispatchResult(DispatcherServlet.java:1012)
        at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:959)
        at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:876)
        at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:931)
        at org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:833)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:646)
        at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:807)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:727)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:303)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
        at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
        at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:220)
        at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:122)
        at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:501)
        at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:170)
        at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:98)
        at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:950)
        at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:116)
        at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:408)
        at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1040)
        at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:607)
        at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:313)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
        at java.lang.Thread.run(Unknown Source)
    Caused by: java.lang.IllegalArgumentException: 'items' must not be null
        at org.springframework.util.Assert.notNull(Assert.java:112)
        at org.springframework.web.servlet.tags.form.AbstractMultiCheckedElementTag.setItems(AbstractMultiCheckedElementTag.java:85)
        at org.apache.jsp.WEB_002dINF.pages.addstudent_jsp._jspx_meth_form_005fradiobuttons_005f0(addstudent_jsp.java:810)
        at org.apache.jsp.WEB_002dINF.pages.addstudent_jsp._jspService(addstudent_jsp.java:360)
        at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:727)
        at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:432)
        ... 42 more
    Jun 26, 2014 4:32:00 PM org.apache.catalina.core.StandardWrapperValve invoke
    SEVERE: Servlet.service() for servlet [mvc-dispatcher] in context with path [/sms] threw exception [An exception occurred processing JSP page /WEB-INF/pages/addstudent.jsp at line 62

    59:             </tr>
    60:             <tr>
    61:                 <td>Gender:</td>
    62:                 <td><form:radiobuttons path="gender" items="${genderList}" />
    63:                 </td>
    64:                 <td><form:errors path="gender" cssClass="error" />
    65:             </tr>


    Stacktrace:] with root cause
    java.lang.IllegalArgumentException: 'items' must not be null
        at org.springframework.util.Assert.notNull(Assert.java:112)
        at org.springframework.web.servlet.tags.form.AbstractMultiCheckedElementTag.setItems(AbstractMultiCheckedElementTag.java:85)
        at org.apache.jsp.WEB_002dINF.pages.addstudent_jsp._jspx_meth_form_005fradiobuttons_005f0(addstudent_jsp.java:810)
        at org.apache.jsp.WEB_002dINF.pages.addstudent_jsp._jspService(addstudent_jsp.java:360)
        at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:727)
        at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:432)
        at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:390)
        at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:334)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:727)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:303)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
        at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
        at org.apache.catalina.core.ApplicationDispatcher.invoke(ApplicationDispatcher.java:748)
        at org.apache.catalina.core.ApplicationDispatcher.processRequest(ApplicationDispatcher.java:486)
        at org.apache.catalina.core.ApplicationDispatcher.doForward(ApplicationDispatcher.java:411)
        at org.apache.catalina.core.ApplicationDispatcher.forward(ApplicationDispatcher.java:338)
        at org.springframework.web.servlet.view.InternalResourceView.renderMergedOutputModel(InternalResourceView.java:209)
        at org.springframework.web.servlet.view.AbstractView.render(AbstractView.java:266)
        at org.springframework.web.servlet.DispatcherServlet.render(DispatcherServlet.java:1225)
        at org.springframework.web.servlet.DispatcherServlet.processDispatchResult(DispatcherServlet.java:1012)
        at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:959)
        at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:876)
        at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:931)
        at org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:833)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:646)
        at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:807)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:727)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:303)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
        at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
        at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:220)
        at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:122)
        at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:501)
        at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:170)
        at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:98)
        at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:950)
        at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:116)
        at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:408)
        at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1040)
        at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:607)
        at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:313)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
        at java.lang.Thread.run(Unknown Source)

我能够弄清楚，我用于指定性别、gradeList、extraSubjects 的List 在发布请求后似乎是NULL。

如果验证失败，我如何才能在发布后再次在表单中呈现这些值？
我是否也必须像在 GET 方法中一样在 POST 方法的列表中设置值？

最佳答案

在你得到部分，你填充你的模型，不仅有 studentForm 还有 genderList, gradeList 和 extraSubjects。你在你的 jsp 中使用了所有。

在帖子部分，您有一个由请求中的参数初始化的studentForm。 但您永远不会初始化列表。所以在JSP中，${gradeList}、${genderList}和${extraSubjects}都是null，所以报错。

如果验证失败，则必须在转发到 View 之前初始化 addStudent 中的三个列表。您可以按照 Neil McGuigan 的建议来做到这一点，即让 spring 自动填充模型，或者您自己填充它。例如，您可以:

void initLists(Model model) {
    LinkedHashMap<Integer, String> gradeList = new LinkedHashMap<Integer, String>();
    gradeList.put(1, "Class 1");
    gradeList.put(2, "Class 2");
    gradeList.put(3, "Class 3");
    gradeList.put(4, "Class 4");
    gradeList.put(5, "Class 5");
    model.addAttribute("gradeList", gradeList);

    List<String> genderList = new ArrayList<String>();
    genderList.add("Male");
    genderList.add("Female");
    model.addAttribute("genderList", genderList);

    List<String> extraSubjects = new ArrayList<String>();
    extraSubjects.add("Math II");
    extraSubjects.add("Population");
    extraSubjects.add("Computer");
    model.addAttribute("extraSubjects", extraSubjects);
}

并在 addStudentForm 和 addStudent 中调用 initLists。最后的示例代码:

@RequestMapping(value = "/add", method = RequestMethod.POST)
public String addStudent(
        @Valid @ModelAttribute("studentForm") Student student,
        BindingResult result, Model model) {

    if (result.hasErrors()) {
        logger.info("Returning add page with errors");
        initLists(model);
        return "addstudent";
    }
        //Stuffs to save in database 
    return "redirect:add";
}

当然，一旦您从那里调用 initLists，就应该从 addStudentForm 中删除所有列表初始化...

关于java - Spring 表单验证错误 : java. lang.IllegalArgumentException: 'items' must not be null，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/24428930/

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我主要有应用程序开发背景。在编程语言中 variable == null或 variable != null有效。当涉及到 SQL 时，以下查询不会给出任何语法错误，但也不会返回正确的结果。 sel
null - NULL 检查中的段错误？
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sql - 强制 COALESE(NULL,NULL) 返回 NULL
在 SQL 服务器中考虑到以下事实:Col1 和 Col2 包含数值和 NULL 值 SELECT COALESCE(Col1,Col2) 返回一个错误:“COALESCE 的至少一个参数必须是一个不
sql - 强制 COALESE(NULL,NULL) 返回 NULL
在 SQL 服务器中考虑到以下事实:Col1 和 Col2 包含数值和 NULL 值 SELECT COALESCE(Col1,Col2) 返回一个错误:“COALESCE 的至少一个参数必须是一个不
null - "is null"的关系代数表达式
下面查询的关系代数表达式是什么？我找不到“Is Null”的表达式。 SELECT reader.name FROM reader LEFT JOIN book_borrow ON reader.ca
c# - 使用三元运算符检查 null - null 引用是否与 null 不同
我正在尝试使用三元运算符来检查值是否为 null 并返回一个表达式或另一个。将此合并到 LINQ 表达式时，我遇到的是 LINQ 表达式的 Transact-SQL 转换试图执行“column = n
c++ - select(NULL, NULL, NULL, &timeout) 只是等待给定的时间吗？
我在给定的代码中看到了以下行: select(0, (fd_set *) NULL, (fd_set *) NULL, (fd_set *) NULL, &timeout); http://linux
javascript -/null/g.test ('null null' 的意外正则表达式行为)
var re = /null/g; re.test('null null'); //> true re.test('null null'); //> true re.test('null null')
null - null 有害吗？
这个问题在这里已经有了答案: 关闭 13 年前。我今天避开了一场关于数据库中空值的激烈辩论。我的观点是 null 是未指定值的极好指示符。团队中有意见的其他每个人都认为零和空字符串是可行的方法。
java - 错误:null value in entry: streamOutputFolder=null OR Error:null value in entry: streamOutputFolder=null
由于此错误，我无法在模拟器中运行我的应用: Error:null value in entry: streamOutputFolder=null 或 gradle - Error:null value
java - 无法解析 'query(java.lang.String,java.lang.String[],null, null, null, null)'
我正在尝试在 Android 应用程序中创建电影数据库，但它返回错误。知道这意味着什么吗？ public Cursor returnData() { return db.query(TABLE
javascript - 为什么 new Date(null, null, null) 在浏览器中返回有效日期？
我一直在检查浏览器中的日期函数以及运行时间 new Date (null, null, null); 在开发工具控制台中，它给出了有效的日期 Chrome v 61 回归 Sun Dec 31 189
r - 为什么 NA==NULL ， NULL==NULL 打印逻辑(0)？
为什么 NA==NULL 会导致 logical (0) 而不是 FALSE？为什么 NULL==NULL 会导致 logical(0) 而不是 TRUE？最佳答案 NULL 是一个“零长度”对象

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java - Spring 表单验证错误 : java. lang.IllegalArgumentException: 'items' must not be null