java - 为什么当我添加productimage代码和 "productname"ProductNAME时不支持 "NULL not allowed for column "; SQL语句:"-6ren

java - 为什么当我添加productimage代码和 "productname"ProductNAME时不支持 "NULL not allowed for column "; SQL语句:"

转载作者：行者123 更新时间：2023-12-02 11:56:46

   i am not add any null in "productName" but error   org.hibernate.exception.ConstraintViolationException: NULL not allowed for column "PRODUCTNAME"; SQL statement

我的错误 null not allowed Productname 那么我该如何解决这个问题 HTTP Status 500 - 请求处理失败；嵌套异常是 org.hibernate.exception.ConstraintViolationException:列“ProductNAME”不允许为 NULL； SQL语句:

    org.hibernate.engine.jdbc.spi.SqlExceptionHelper.logExceptions SQL Error: 23502, SQLState: 23502

 org.hibernate.engine.jdbc.spi.SqlExceptionHelper.logExceptions NULL not allowed for column "PRODUCTNAME"; SQL statement

这是完整的错误:

   type: Exception report

        message Request processing failed; nested exception is org.hibernate.exception.ConstraintViolationException: NULL not allowed for column "PRODUCTNAME"; SQL statement:

        description The server encountered an internal error that prevented it from fulfilling this request.

        exception

        org.springframework.web.util.NestedServletException: Request processing failed; nested exception is org.hibernate.exception.ConstraintViolationException: NULL not allowed for column "PRODUCTNAME"; SQL statement:
        insert into Product (productId, productCategory, productCondition, productDescription, productManufacturer, productName, productPrice, productStatus, unitInStock) values (null, ?, ?, ?, ?, ?, ?, ?, ?) [23502-196]
            org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:978)
            org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:868)
            javax.servlet.http.HttpServlet.service(HttpServlet.java:648)
            org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:842)
            javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
            org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
        root cause

        org.hibernate.exception.ConstraintViolationException: NULL not allowed for column "PRODUCTNAME"; SQL statement:
        insert into Product (productId, productCategory, productCondition, productDescription, productManufacturer, productName, productPrice, productStatus, unitInStock) values (null, ?, ?, ?, ?, ?, ?, ?, ?) [23502-196]
            org.hibernate.exception.internal.SQLStateConversionDelegate.convert(SQLStateConversionDelegate.java:128)
            org.hibernate.exception.internal.StandardSQLExceptionConverter.convert(StandardSQLExceptionConverter.java:47)
            org.hibernate.engine.jdbc.spi.SqlExceptionHelper.convert(SqlExceptionHelper.java:125)
            org.hibernate.engine.jdbc.spi.SqlExceptionHelper.convert(SqlExceptionHelper.java:110)
            org.hibernate.engine.jdbc.internal.proxy.AbstractStatementProxyHandler.continueInvocation(AbstractStatementProxyHandler.java:129)
            org.hibernate.engine.jdbc.internal.proxy.AbstractProxyHandler.invoke(AbstractProxyHandler.java:81)
            com.sun.proxy.$Proxy47.executeUpdate(Unknown Source)
            org.hibernate.id.IdentityGenerator$GetGeneratedKeysDelegate.executeAndExtract(IdentityGenerator.java:96)
            org.hibernate.id.insert.AbstractReturningDelegate.performInsert(AbstractReturningDelegate.java:55)
            org.hibernate.persister.entity.AbstractEntityPersister.insert(AbstractEntityPersister.java:2757)
            org.hibernate.persister.entity.AbstractEntityPersister.insert(AbstractEntityPersister.java:3268)
            org.hibernate.action.internal.EntityIdentityInsertAction.execute(EntityIdentityInsertAction.java:78)
            org.hibernate.engine.spi.ActionQueue.execute(ActionQueue.java:272)
            org.hibernate.event.internal.AbstractSaveEventListener.performSaveOrReplicate(AbstractSaveEventListener.java:304)
            org.hibernate.event.internal.AbstractSaveEventListener.performSave(AbstractSaveEventListener.java:195)
            org.hibernate.event.internal.AbstractSaveEventListener.saveWithGeneratedId(AbstractSaveEventListener.java:128)
            org.hibernate.event.internal.DefaultSaveOrUpdateEventListener.saveWithGeneratedOrRequestedId(DefaultSaveOrUpdateEventListener.java:204)
            org.hibernate.event.internal.DefaultSaveOrUpdateEventListener.entityIsTransient(DefaultSaveOrUpdateEventListener.java:189)
            org.hibernate.event.internal.DefaultSaveOrUpdateEventListener.performSaveOrUpdate(DefaultSaveOrUpdateEventListener.java:114)
            org.hibernate.event.internal.DefaultSaveOrUpdateEventListener.onSaveOrUpdate(DefaultSaveOrUpdateEventListener.java:90)
            org.hibernate.internal.SessionImpl.fireSaveOrUpdate(SessionImpl.java:615)
            org.hibernate.internal.SessionImpl.saveOrUpdate(SessionImpl.java:608)
            org.hibernate.internal.SessionImpl.saveOrUpdate(SessionImpl.java:604)
            com.emusicstore.dao.impl.ProductDaoImpl.addProduct(ProductDaoImpl.java:25)
            sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
            sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
            sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
            java.lang.reflect.Method.invoke(Method.java:497)`enter code here`
            org.springframework.aop.support.AopUtils.invokeJoinpointUsingReflection(AopUtils.java:317)
            org.springframework.aop.framework.ReflectiveMethodInvocation.invokeJoinpoint(ReflectiveMethodInvocation.java:190)
            org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:157)
            org.springframework.transaction.interceptor.TransactionInterceptor$1.proceedWithInvocation(TransactionInterceptor.java:99)
            org.springframework.transaction.interceptor.TransactionAspectSupport.invokeWithinTransaction(TransactionAspectSupport.java:281)
            org.springframework.transaction.interceptor.TransactionInterceptor.invoke(TransactionInterceptor.java:96)
            org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:179)
            org.springframework.aop.framework.JdkDynamicAopProxy.invoke(JdkDynamicAopProxy.java:207)
            com.sun.proxy.$Proxy42.addProduct(Unknown Source)
            com.emusicstore.controller.HomeController.addProductPost(HomeController.java:84)
            sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
            sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
            sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
            java.lang.reflect.Method.invoke(Method.java:497)
            org.springframework.web.method.support.InvocableHandlerMethod.doInvoke(InvocableHandlerMethod.java:221)
            org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:137)
            org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:110)
            org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandleMethod(RequestMappingHandlerAdapter.java:777)
            org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:706)
            org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:85)
            org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:943)
            org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:877)
            org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:966)
            org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:868)
            javax.servlet.http.HttpServlet.service(HttpServlet.java:648)
            org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:842)
            javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
            org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
        root cause

        org.h2.jdbc.JdbcSQLException: NULL not allowed for column "PRODUCTNAME"; SQL statement:
        insert into Product (productId, productCategory, productCondition, productDescription, productManufacturer, productName, productPrice, productStatus, unitInStock) values (null, ?, ?, ?, ?, ?, ?, ?, ?) [23502-196]
            at org.h2.message.DbException.getJdbcSQLException(DbException.java:345)
            at org.h2.message.DbException.get(DbException.java:179)
            at org.h2.message.DbException.get(DbException.java:155)
            at org.h2.table.Column.validateConvertUpdateSequence(Column.java:345)
            at org.h2.table.Table.validateConvertUpdateSequence(Table.java:797)
            at org.h2.command.dml.Insert.insertRows(Insert.java:151)
            at org.h2.command.dml.Insert.update(Insert.java:114)
            at org.h2.command.CommandContainer.update(CommandContainer.java:101)
            at org.h2.command.Command.executeUpdate(Command.java:260)
            at org.h2.server.TcpServerThread.process(TcpServerThread.java:354)
            at org.h2.server.TcpServerThread.run(TcpServerThread.java:158)
            at java.lang.Thread.run(Unknown Source)

            org.h2.engine.SessionRemote.done(SessionRemote.java:629)
            org.h2.command.CommandRemote.executeUpdate(CommandRemote.java:209)
            org.h2.jdbc.JdbcPreparedStatement.executeUpdateInternal(JdbcPreparedStatement.java:164)
            org.h2.jdbc.JdbcPreparedStatement.executeUpdate(JdbcPreparedStatement.java:150)
            sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
            sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
            sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
            java.lang.reflect.Method.invoke(Method.java:497)
            org.hibernate.engine.jdbc.internal.proxy.AbstractStatementProxyHandler.continueInvocation(AbstractStatementProxyHandler.java:122)
            org.hibernate.engine.jdbc.internal.proxy.AbstractProxyHandler.invoke(AbstractProxyHandler.java:81)
            com.sun.proxy.$Proxy47.executeUpdate(Unknown Source)
            org.hibernate.id.IdentityGenerator$GetGeneratedKeysDelegate.executeAndExtract(IdentityGenerator.java:96)
            org.hibernate.id.insert.AbstractReturningDelegate.performInsert(AbstractReturningDelegate.java:55)
            org.hibernate.persister.entity.AbstractEntityPersister.insert(AbstractEntityPersister.java:2757)
            org.hibernate.persister.entity.AbstractEntityPersister.insert(AbstractEntityPersister.java:3268)
            org.hibernate.action.internal.EntityIdentityInsertAction.execute(EntityIdentityInsertAction.java:78)
            org.hibernate.engine.spi.ActionQueue.execute(ActionQueue.java:272)
            org.hibernate.event.internal.AbstractSaveEventListener.performSaveOrReplicate(AbstractSaveEventListener.java:304)
            org.hibernate.event.internal.AbstractSaveEventListener.performSave(AbstractSaveEventListener.java:195)
            org.hibernate.event.internal.AbstractSaveEventListener.saveWithGeneratedId(AbstractSaveEventListener.java:128)
            org.hibernate.event.internal.DefaultSaveOrUpdateEventListener.saveWithGeneratedOrRequestedId(DefaultSaveOrUpdateEventListener.java:204)
            org.hibernate.event.internal.DefaultSaveOrUpdateEventListener.entityIsTransient(DefaultSaveOrUpdateEventListener.java:189)
            org.hibernate.event.internal.DefaultSaveOrUpdateEventListener.performSaveOrUpdate(DefaultSaveOrUpdateEventListener.java:114)
            org.hibernate.event.internal.DefaultSaveOrUpdateEventListener.onSaveOrUpdate(DefaultSaveOrUpdateEventListener.java:90)
            org.hibernate.internal.SessionImpl.fireSaveOrUpdate(SessionImpl.java:615)
            org.hibernate.internal.SessionImpl.saveOrUpdate(SessionImpl.java:608)
            org.hibernate.internal.SessionImpl.saveOrUpdate(SessionImpl.java:604)
            com.emusicstore.dao.impl.ProductDaoImpl.addProduct(ProductDaoImpl.java:25)
            sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
            sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
            sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
            java.lang.reflect.Method.invoke(Method.java:497)
            org.springframework.aop.support.AopUtils.invokeJoinpointUsingReflection(AopUtils.java:317)
            org.springframework.aop.framework.ReflectiveMethodInvocation.invokeJoinpoint(ReflectiveMethodInvocation.java:190)
            org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:157)
            org.springframework.transaction.interceptor.TransactionInterceptor$1.proceedWithInvocation(TransactionInterceptor.java:99)
            org.springframework.transaction.interceptor.TransactionAspectSupport.invokeWithinTransaction(TransactionAspectSupport.java:281)
            org.springframework.transaction.interceptor.TransactionInterceptor.invoke(TransactionInterceptor.java:96)
            org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:179)
            org.springframework.aop.framework.JdkDynamicAopProxy.invoke(JdkDynamicAopProxy.java:207)
            com.sun.proxy.$Proxy42.addProduct(Unknown Source)
            com.emusicstore.controller.HomeController.addProductPost(HomeController.java:84)
            sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
            sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
            sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
            java.lang.reflect.Method.invoke(Method.java:497)
            org.springframework.web.method.support.InvocableHandlerMethod.doInvoke(InvocableHandlerMethod.java:221)
            org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:137)
            org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:110)
            org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandleMethod(RequestMappingHandlerAdapter.java:777)
            org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:706)
            org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:85)
            org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:943)
            org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:877)
            org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:966)
            org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:868)
            javax.servlet.http.HttpServlet.service(HttpServlet.java:648)
            org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:842)
            javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
            org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
        note The full stack trace of the root cause is available in the Apache Tomcat/8.0.28 logs.

我的添加产品页面:

这是添加产品页面代码

    <%@taglib prefix="form" uri="http://www.springframework.org/tags/form" %>
    <%@include file="/WEB-INF/views/template/header.jsp"%>


    <div class="container-wrapper">
        <div class="container">
            <div class="page-header">
                <h1>Add Product</h1>
                <p class="lead">Fill the below information to a  product: </p>
            </div>


          <form:form action="${pageContext.request.contextPath}/admin/productInventory/addProduct"  method="post"  commandName="product" enctype="multipart/form-data">

            <div class="form-group">
                <label for="name">Name</label>
                <form:input path="productName" id="name" class="form-Control"/>
            </div>

            <div class="form-group">
                <label for="category">Category</label>
                <label class="checkbox-inline"><form:radiobutton path="productCategory" id="category" value="instrument"/>Instrument</label>
                <label class="checkbox-inline"><form:radiobutton path="productCategory" id="category" value="record"/>Record</label>
                <label class="checkbox-inline"><form:radiobutton path="productCategory" id="category" value="accessory"/>Accessory</label>

            </div>

            <div class="form-group">
                <label for="description">Description</label>
                <form:textarea path="productDescription" id="description" class="form-Control"/>
            </div>

            <div class="form-group">
                <label for="price">Price</label>
                <form:input path="productPrice" id="price" class="form-Control"/>
            </div>

            <div class="form-group">
                <label for="condition">Condition</label>
                <label class="checkbox-inline"><form:radiobutton path="productCondition" id="condition" value="new"/>New</label>
                <label class="checkbox-inline"><form:radiobutton path="productCondition" id="condition" value="used"/>Used</label>

            </div>



            <div class="form-group">
                <label for="status">Status</label>
                <label class="checkbox-inline"><form:radiobutton path="productStatus" id="status" value="active"/>Active</label>
                <label class="checkbox-inline"><form:radiobutton path="productStatus" id="status" value="inactive"/>Inactive</label>

            </div>

            <div class="form-group">
                <label for="unitInstock">Unit In stock</label>
                <form:input path="unitInStock" id="unitInstock" class="form-Control"/>
            </div>

            <div class="form-group">
                <label for="manufacturer">Manufacturer</label>
                <form:input path="productManufacturer" id="manufacturer" class="form-Control"/>
            </div>

            <div class="form-group">
                <label class="control-label" for="productImage">Upload Picture</label>
                <form:input id="productImage" path="productImage" type="file" class="form:input-large"/>

            </div>

            <br><br>

            <input type="submit" value="submit" class="btn btn-success">
            <a href="<c:url value="/admin/productInventory"/> " class="btn btn-default">Cancle</a>



            </form:form>
    <%@include file="/WEB-INF/views/template/footer.jsp"%>

我的 Controller 映射:

这是产品图像代码

    //productimage code
    @RequestMapping(value = "/admin/productInventory/addProduct" , method = RequestMethod.POST)
        public String addProductPost(@ModelAttribute("product") Product product, HttpServletRequest request){

            productDao.addProduct(product);

            MultipartFile productImage = product.getProductImage();
            String rootDirectory = request.getSession().getServletContext().getRealPath("/");
            path = Paths.get(rootDirectory + "\\WEB-INF\\resources\\images\\"+product.getProductId()+".png");

            if (productImage !=null && !productImage.isEmpty())
            {
                try{
                    productImage.transferTo(new File(path.toString()));
                }catch (Exception e)
                {
                    e.printStackTrace();
                    throw new RuntimeException("Product image saving failed",e);

                }
            }

            return "redirect:/admin/productInventory";

        }
   productimage//

我的模型Product.java

    this is add our product column

    package com.emusicstore.model;

    import org.springframework.web.multipart.MultipartFile;

    import javax.persistence.*;

    @Entity

    public class Product {

        @Id
        @GeneratedValue(strategy = GenerationType.AUTO)

        private String productId;
        private String productName;
        private String productCategory;
        private String productDescription;
        private double productPrice;
        private String productCondition;
        private String productStatus;
        private int unitInStock;
        private String productManufacturer;

        @Transient//automatic create productImage in database
        private MultipartFile productImage;

        public String getProductId() {
            return productId;
        }

        public void setProductId(String productId) {
            this.productId = productId;
        }



        public String getProductName() {
            return productName;
        }

        public void setProductName(String productName) {
            this.productName = productName;
        }

        public String getProductCategory() {
            return productCategory;
        }

        public void setProductCategory(String productCategory) {
            this.productCategory = productCategory;
        }

        public String getProductDescription() {
            return productDescription;
        }

        public void setProductDescription(String productDescription) {
            this.productDescription = productDescription;
        }

        public double getProductPrice() {
            return productPrice;
        }

        public void setProductPrice(double productPrice) {
            this.productPrice = productPrice;
        }

        public String getProductCondition() {
            return productCondition;
        }

        public void setProductCondition(String productCondition) {
            this.productCondition = productCondition;
        }

        public String getProductStatus() {
            return productStatus;
        }

        public void setProductStatus(String productStatus) {
            this.productStatus = productStatus;
        }

        public int getUnitInStock() {
            return unitInStock;
        }

        public void setUnitInStock(int unitInStock) {
            this.unitInStock = unitInStock;
        }

        public String getProductManufacturer() {
            return productManufacturer;
        }

        public void setProductManufacturer(String productManufacturer) {
            this.productManufacturer = productManufacturer;
        }


        public MultipartFile getProductImage() {
            return productImage;
        }

        public void setProductImage(MultipartFile productImage) {
            this.productImage = productImage;
        }
    }

我的产品daoimpl:

    package com.emusicstore.dao.impl;

    import com.emusicstore.dao.ProductDao;
    import com.emusicstore.model.Product;
    import org.hibernate.Query;
    import org.hibernate.Session;
    import org.hibernate.SessionFactory;
    import org.springframework.beans.factory.annotation.Autowired;
    import org.springframework.stereotype.Repository;
    import org.springframework.transaction.annotation.Transactional;

    import java.util.List;


    @Repository
    @Transactional
    public class ProductDaoImpl implements ProductDao {


        @Autowired
        private SessionFactory sessionFactory;

        public void addProduct(Product product){
            Session session = sessionFactory.getCurrentSession();
            session.saveOrUpdate(product);
            session.flush();
        }

        public Product getProductById(String id){
            Session session = sessionFactory.getCurrentSession();
            Product product = (Product) session.get(Product.class,id);
            session.flush();
            return product;
        }

        public List<Product> getAllProducts(){
            Session session = sessionFactory.getCurrentSession();
            Query query = session.createQuery("from Product");
            List<Product> products= query.list();
            session.flush();

            return products;
        }

        public void deleteProduct(String id){
            Session session = sessionFactory.getCurrentSession();
            session.delete(getProductById(id));
            session.flush();


        }
    }

pom.xml for image
-----------------

这是我们的图像 pom.xml

  <dependency>
            <groupId>commons-fileupload</groupId>
            <artifactId>commons-fileupload</artifactId>
            <version>1.2.2</version>
        </dependency>

        <dependency>
            <groupId>commons-io</groupId>
            <artifactId>commons-io</artifactId>
            <version>2.4</version>
        </dependency>

        <dependency>
            <groupId>javax.servlet</groupId>
            <artifactId>servlet-api</artifactId>
            <version>2.5</version>
            <scope>provided</scope>
        </dependency>

实际上，错误是什么，导致我无法将文件插入数据库，请帮助我

最佳答案

您要保存的产品的 PRODUCTNAME 属性值为空。在调用函数saveOrUpdate之前确保它不为空或从数据库中的表中删除not null约束。

关于java - 为什么当我添加productimage代码和 "productname"ProductNAME时不支持 "NULL not allowed for column "; SQL语句:"，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/47556746/

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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
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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)
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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
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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
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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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