为什么我删除分类列表时，也会把里边商品删除？

package com.imooc.service.impl;

import java.sql.Connection;

import java.sql.SQLException;

import java.util.List;

import com.imooc.dao.CategoryDao;

import com.imooc.dao.ProductDao;

import com.imooc.dao.impl.CategoryDaoImpl;

import com.imooc.dao.impl.ProductDaoImpl;

import com.imooc.domain.Category;

import com.imooc.domain.Product;

/**

 * 分类的Serive实现接口

 */

import com.imooc.service.CategoryService;

import com.imooc.service.ProductService;

import com.imooc.utils.JDBCUtils;

public class CategoryServiceImpl implements CategoryService {

@Override

public List<Category> findAll() {

System.out.println("CategoryService的findAll方法执行了");

CategoryDao categoryDao=new CategoryDaoImpl();

return categoryDao.findAll();

}

@Override

public void save(Category category) {

CategoryDao CategoryDao=new CategoryDaoImpl();

CategoryDao.save(category);

}

@Override

public Category findOne(Integer cid) {

CategoryDao categoryDao = new CategoryDaoImpl();

return categoryDao.findOne(cid);

}

@Override

public void update(Category category) {

CategoryDao categoryDao=new CategoryDaoImpl();

categoryDao.update(category);

}

@Override

public void delete(Integer cid) {

/**

* 事务管理：在业务层统一创建连接对象，保证多个DAO中使用同一个连接

* 1.创建连接以后，将连接对象传给DAO

* 2.创建一个连接对象，将连接绑定到线程中（ThreadLocal）

* */

Connection conn=null;

try {

// 获得连接

conn=JDBCUtils.getConnection();

//开启事务：把自动提交关闭

conn.setAutoCommit(false);

// 要在删除分类之前，先将所属该分类的商品处理一下

ProductDao productDao = new ProductDaoImpl();

List<Product> list = productDao.finByCid(cid);

// 将查询出分类的商品设为NULL

for (Product product : list) {

product.getCategory().setCid(null);

productDao.update(conn,product);

}

// 删除分类

CategoryDao categoryDao = new CategoryDaoImpl();

categoryDao.delete(conn,cid);

//提交事务：手动提交

conn.commit();

} catch (Exception e) {

//回滚事务：

try {

conn.rollback();

} catch (SQLException e1) {

e1.printStackTrace();

}

e.printStackTrace();

} finally {

if(conn != null) {

try {

conn.close();

} catch (SQLException e) {

// TODO Auto-generated catch block

e.printStackTrace();

}

conn =null;

}

}

}

}

package com.imooc.dao.impl;

import java.sql.Connection;

import java.sql.PreparedStatement;

import java.sql.ResultSet;

import java.sql.SQLException;

import java.util.ArrayList;

import java.util.List;

import com.imooc.dao.ProductDao;

import com.imooc.domain.Product;

import com.imooc.utils.JDBCUtils;

/**

 * 商品实现类实现接口

 * @author 14628

 *

 */

public class ProductDaoImpl implements ProductDao{

@Override

public List<Product> findAll() {

System.out.println("ProductDao的findAll方法执行了");

Connection conn=null;

PreparedStatement psta=null;

ResultSet rs=null;

List<Product> list=null;

try {

//获得连接

conn=JDBCUtils.getConnection();

//编译SQL

String sql="SELECT * FROM product p,category c WHERE p.cid=c.cid ORDER BY p.pid DESC";

//预编译SQL

psta=conn.prepareStatement(sql);

//执行SQL

rs=psta.executeQuery();

//处理结果集

list=new ArrayList<Product>();

while(rs.next()) {

Product product =new Product();

product.setPid(rs.getInt("pid"));

product.setPname(rs.getString("pname"));

product.setAuthor(rs.getString("author"));

product.setPrice(rs.getDouble("price"));

product.setDescription(rs.getString("description"));

product.setFilename(rs.getString("filename"));

product.setPath(rs.getString("path"));

//封装商品所属的分类

product.getCategory().setCid(rs.getInt("cid"));

product.getCategory().setCname(rs.getString("cname"));

product.getCategory().setCdesc(rs.getString("cdesc"));

list.add(product);

}

}catch(Exception e) {

e.printStackTrace();

}finally {

JDBCUtils.release(rs, psta, conn);

}

return list;

}

@Override

public void save(Product product) {

Connection conn=null;

PreparedStatement psta=null;

try {

//获得连接

conn=JDBCUtils.getConnection();

String sql="INSERT INTO product VALUES(null,?,?,?,?,?,?,?)";

psta=conn.prepareStatement(sql);

psta.setString(1, product.getPname());

psta.setString(2, product.getAuthor());

psta.setDouble(3, product.getPrice());

psta.setString(4, product.getDescription());

psta.setString(5, product.getFilename());

psta.setString(6, product.getPath());

psta.setInt(7, product.getCategory().getCid());

psta.executeUpdate();

}catch(Exception e) {

e.printStackTrace();

}finally {

JDBCUtils.release(psta, conn);

}

}

@Override

public Product findOne(Integer pid) {

Connection conn=null;

PreparedStatement psta=null;

ResultSet rs=null;

try {

conn=JDBCUtils.getConnection();

String sql="SELECT * FROM product p,category c WHERE p.cid=c.cid AND p.pid=?";

psta=conn.prepareStatement(sql);

psta.setInt(1, pid);

rs=psta.executeQuery();

if(rs.next()) {

Product product=new Product();

product.setPid(rs.getInt("pid"));

product.setPname(rs.getString("pname"));

product.setAuthor(rs.getString("author"));

product.setPrice(rs.getDouble("price"));

product.setDescription(rs.getString("description"));

product.setFilename(rs.getString("filename"));

product.setPath(rs.getString("path"));

product.getCategory().setCid(rs.getInt("cid"));

product.getCategory().setCname(rs.getString("cname"));

product.getCategory().setCdesc(rs.getString("cdesc"));

return product;

}

}catch(Exception e) {

e.printStackTrace();

}finally {

JDBCUtils.release(rs,psta, conn);

}

return null;

}

@Override

public void update(Product product) {

Connection conn=null;

PreparedStatement psta=null;

try {

//获得连接

conn=JDBCUtils.getConnection();

String sql="UPDATE product SET pname=?,author=?,price=?,description=?,filename=?,path=?,cid=? WHERE pid=?";

psta=conn.prepareStatement(sql);

psta.setString(1, product.getPname());

psta.setString(2, product.getAuthor());

psta.setDouble(3, product.getPrice());

psta.setString(4, product.getDescription());

psta.setString(5, product.getFilename());

psta.setString(6, product.getPath());

psta.setObject(7,product.getCategory().getCid());

psta.setInt(8, product.getPid());

psta.executeUpdate();

}catch(Exception e) {

e.printStackTrace();

}finally {

JDBCUtils.release(psta, conn);

}

}

@Override

public void delete(Integer pid) {

Connection conn=null;

PreparedStatement psta=null;

try {

conn=JDBCUtils.getConnection();

String sql="DELETE FROM product WHERE pid=?";

psta=conn.prepareStatement(sql);

psta.setInt(1,pid);

psta.executeUpdate();

}catch(Exception e) {

e.printStackTrace();

}finally {

JDBCUtils.release(psta, conn);

}

}

@Override

public List<Product> finByCid(Integer cid) {

Connection conn=null;

PreparedStatement psta=null;

ResultSet rs=null;

List<Product> list=null;

try {

//获得连接

conn=JDBCUtils.getConnection();

//编译SQL

String sql="SELECT * FROM product p,category c WHERE p.cid=c.cid and p.cid = ? ORDER BY p.pid DESC";

//预编译SQL

psta=conn.prepareStatement(sql);

//设置参数

psta.setInt(1, cid);

//执行SQL

rs=psta.executeQuery();

//处理结果集

list=new ArrayList<Product>();

while(rs.next()) {

Product product =new Product();

product.setPid(rs.getInt("pid"));

product.setPname(rs.getString("pname"));

product.setAuthor(rs.getString("author"));

product.setPrice(rs.getDouble("price"));

product.setDescription(rs.getString("description"));

product.setFilename(rs.getString("filename"));

product.setPath(rs.getString("path"));

//封装商品所属的分类

product.getCategory().setCid(rs.getInt("cid"));

product.getCategory().setCname(rs.getString("cname"));

product.getCategory().setCdesc(rs.getString("cdesc"));

list.add(product);

}

}catch(Exception e) {

e.printStackTrace();

}finally {

JDBCUtils.release(rs, psta, conn);

}

return list;

}

//重写update方法，在业务层中获得连接

@Override

public void update(Connection conn, Product product) {

PreparedStatement psta=null;

try {

String sql="UPDATE product SET pname=?,author=?,price=?,description=?,filename=?,path=?,cid=? WHERE pid=?";

psta=conn.prepareStatement(sql);

psta.setString(1, product.getPname());

psta.setString(2, product.getAuthor());

psta.setDouble(3, product.getPrice());

psta.setString(4, product.getDescription());

psta.setString(5, product.getFilename());

psta.setString(6, product.getPath());

psta.setObject(7,product.getCategory().getCid());

psta.setInt(8, product.getPid());

psta.executeUpdate();

}catch(Exception e) {

e.printStackTrace();

}finally {

if(psta != null) {

try {

psta.close();

} catch (SQLException e) {

// TODO Auto-generated catch block

e.printStackTrace();

}

psta =null;

}

}

}

}

点击删除报错：

java.sql.SQLIntegrityConstraintViolationException: Column 'cid' cannot be null

at com.mysql.cj.jdbc.exceptions.SQLError.createSQLException(SQLError.java:117)

at com.mysql.cj.jdbc.exceptions.SQLError.createSQLException(SQLError.java:97)

at com.mysql.cj.jdbc.exceptions.SQLExceptionsMapping.translateException(SQLExceptionsMapping.java:122)

at com.mysql.cj.jdbc.ClientPreparedStatement.executeInternal(ClientPreparedStatement.java:955)

at com.mysql.cj.jdbc.ClientPreparedStatement.executeUpdateInternal(ClientPreparedStatement.java:1094)

at com.mysql.cj.jdbc.ClientPreparedStatement.executeUpdateInternal(ClientPreparedStatement.java:1042)

at com.mysql.cj.jdbc.ClientPreparedStatement.executeLargeUpdate(ClientPreparedStatement.java:1345)

at com.mysql.cj.jdbc.ClientPreparedStatement.executeUpdate(ClientPreparedStatement.java:1027)

at com.mchange.v2.c3p0.impl.NewProxyPreparedStatement.executeUpdate(NewProxyPreparedStatement.java:105)

at com.imooc.dao.impl.ProductDaoImpl.update(ProductDaoImpl.java:220)

at com.imooc.service.impl.CategoryServiceImpl.delete(CategoryServiceImpl.java:67)

at com.imooc.web.action.CategoryServlet.delete(CategoryServlet.java:165)

at com.imooc.web.action.CategoryServlet.doGet(CategoryServlet.java:47)

at javax.servlet.http.HttpServlet.service(HttpServlet.java:634)

at javax.servlet.http.HttpServlet.service(HttpServlet.java:741)

at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231)

at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)

at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)

at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)

at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)

at com.imooc.web.filter.PrivilegeFilter.doFilter(PrivilegeFilter.java:34)

at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)

at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)

at com.imooc.web.filter.CharacterEncodingFilter.doFilter(CharacterEncodingFilter.java:25)

at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)

at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)

at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:199)

at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:96)

at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:543)

at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:139)

at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:81)

at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:688)

at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:87)

at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:343)

at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:615)

at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:65)

at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:818)

at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1623)

at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49)

at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1135)

at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:635)

at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)

at java.base/java.lang.Thread.run(Thread.java:844)