让MyBatis Generator产生的代码支持分页

孟君

发布于 2020-02-14 16:15:05

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发布于 2020-02-14 16:15:05

文章被收录于专栏：孟君的编程札记

本文提供一种方法，让MyBatis Generator产生的代码支持分页, 适用于MySQL。

▼

分析

如果要获取分页信息，使用MySQL语句，我们需要怎么做呢？

select * from t_user limit 0 , 2

在MySQL系统中，如果要完成一个分页，我们需要指定limit的值，也就是需要指定两个数，第一个指定从什么地方开始（示例中为0）；另一个指定需要获取多少条数据（示例中为2）。

问题转化

如果要使得产生的自动产生的代码具备分页功能的话，那么，Mapper对应的XML中，select语句需要多增加两个属性值，比如：

limitStart （指定从什么位置开始查找）
limitSize （指定找到多少条数据）

上述已经提到需要两个值limitStart和limitSize，那么，我们需要添加在哪里才能有效果呢？

何处添加

以t_news表为例，创建表的SQL语句如下：

CREATE TABLE `t_news` (
  `news_id` int(11) NOT NULL AUTO_INCREMENT,
  `title` varchar(150) NOT NULL,
  `content` text NOT NULL,
  `brief_intro` varchar(255) DEFAULT NULL,
  `pic_url` varchar(255) DEFAULT NULL,
  `news_from` varchar(100) DEFAULT NULL,
  `news_author` varchar(50) DEFAULT NULL,
  `news_url` varchar(255) DEFAULT NULL,
  `keywords` varchar(150) DEFAULT NULL,
  `meta_desc` varchar(150) DEFAULT NULL,
  `create_time` datetime DEFAULT NULL,
  PRIMARY KEY (`news_id`)
) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;

那么，我们可以看到NewsMapper.java中查找列表数据都是通过Example来完成的。

    List<News> selectByExampleWithBLOBs(NewsExample example);

    List<News> selectByExample(NewsExample example);

其中，selectByExampleWithBLOBs方法只有当数据表中的某一列需要存储较大内容的时候，才会产生。来看一下判断是否为BLOB列的源代码吧。摘自IntrospectedColumn类。

    public boolean isBLOBColumn() {
        String typeName = getJdbcTypeName();

        return "BINARY".equals(typeName) || "BLOB".equals(typeName) //$NON-NLS-1$ //$NON-NLS-2$
                || "CLOB".equals(typeName) || "LONGVARBINARY".equals(typeName) //$NON-NLS-1$ //$NON-NLS-2$
                || "LONGVARCHAR".equals(typeName) || "VARBINARY".equals(typeName); //$NON-NLS-1$ //$NON-NLS-2$
    }

注意: 使用selectByExample方法是不会返回BLOB类型的字段，如t_news新闻表中的content内容字段。如果想返回content的值，那么，需要使用selectByExampleWithBLOBs方法。

大家可以来看看如下内容感受一下。selectByExampleWithBLOBs中包含Blob_Column_List，而selectByExample没有。

<select id="selectByExampleWithBLOBs" parameterType="my.mybatis.generator.auto.entity.NewsExample" resultMap="ResultMapWithBLOBs">
    <!--
      WARNING - @mbggenerated
      This element is automatically generated by MyBatis Generator, do not modify.
      This element was generated on Wed Nov 09 19:01:57 CST 2016.
    -->
    select
    <if test="distinct">
      distinct
    </if>
    <include refid="Base_Column_List" />
    ,
    <include refid="Blob_Column_List" />
    from m_news
    <if test="_parameter != null">
      <include refid="Example_Where_Clause" />
    </if>
    <if test="orderByClause != null">
      order by ${orderByClause}
    </if>
  </select>
  <select id="selectByExample" parameterType="my.mybatis.generator.auto.entity.NewsExample" resultMap="BaseResultMap">
    <!--
      WARNING - @mbggenerated
      This element is automatically generated by MyBatis Generator, do not modify.
      This element was generated on Wed Nov 09 19:01:57 CST 2016.
    -->
    select
    <if test="distinct">
      distinct
    </if>
    <include refid="Base_Column_List" />
    from m_news
    <if test="_parameter != null">
      <include refid="Example_Where_Clause" />
    </if>
    <if test="orderByClause != null">
      order by ${orderByClause}
    </if>
  </select>

从上述简单分析可以看出，limitStart和limitSize添加的地方有两个：

实体类对应的Example中需要添加。
XML文件中，selectByExample和selectByExampleWithBLOBs配置需要添加limitStart和limitSize属性。

如

    <if test="limitStart != null and limitSize&gt;=0">
      limit #{limitStart} , #{limitSize}
    </if>

有了上述的分析之后，我们需要写什么就很清楚了。

▼

编码

Example类具有分页属性

Example中包含两个字段limitStart和limitSize，并具有Getter和Setter方法，如：

public class NewsExample {

    protected Integer limitStart;
  
    protected Integer limitSize;

    public void setLimitStart(Integer limitStart) {
        this.limitStart = limitStart;
    }

    public Integer getLimitStart() {
        return limitStart;
    }

    public void setLimitSize(Integer limitSize) {
        this.limitSize = limitSize;
    }

    public Integer getLimitSize() {
        return limitSize;
    }

    //省略其它
}

增加一个私有方法addLimit用于在Example中创建字段并生成Getter和Setter方法：

private void addLimit(TopLevelClass topLevelClass,
      IntrospectedTable introspectedTable, String name) {
    
    CommentGenerator commentGenerator = context.getCommentGenerator();
    
    /**
     * 创建成员变量
     * 如protected Integer limitStart;
     */
    Field field = new Field();
    field.setVisibility(JavaVisibility.PROTECTED);
    field.setType(PrimitiveTypeWrapper.getIntegerInstance());
    field.setName(name);
    commentGenerator.addFieldComment(field, introspectedTable);
    topLevelClass.addField(field);
    /**
     * 首字母大写
     */
    char c = name.charAt(0);
    String camel = Character.toUpperCase(c) + name.substring(1);

    /**
     * 添加Setter方法
     */
    Method method = new Method();
    method.setVisibility(JavaVisibility.PUBLIC);
    method.setName("set" + camel);
    method.addParameter(new Parameter(PrimitiveTypeWrapper
        .getIntegerInstance(), name));

    StringBuilder sb = new StringBuilder();
    sb.append("this.");
    sb.append(name);
    sb.append(" = ");
    sb.append(name);
    sb.append(";");
    /**
     * 如 this.limitStart = limitStart;
     */
    method.addBodyLine(sb.toString());

    commentGenerator.addGeneralMethodComment(method, introspectedTable);
    topLevelClass.addMethod(method);

    /**
     * 添加Getter Method 直接调用AbstractJavaGenerator的getGetter方法
     */
    Method getterMethod = AbstractJavaGenerator.getGetter(field);
    commentGenerator.addGeneralMethodComment(getterMethod,
        introspectedTable);
    topLevelClass.addMethod(getterMethod);
  }

其实，产生上述的代码并不难，因为MyBatis Generator本身就是在为生成的实体类添加变量和Getter Setter方法。

如：

AbstractJavaGenerator抽象类本身就有产生Getter方法的函数，直接调用即可。

public abstract class AbstractJavaGenerator extends AbstractGenerator {
    public abstract List<CompilationUnit> getCompilationUnits();

    public static Method getGetter(Field field) {
        Method method = new Method();
        method.setName(getGetterMethodName(field.getName(), field
                .getType()));
        method.setReturnType(field.getType());
        method.setVisibility(JavaVisibility.PUBLIC);
        StringBuilder sb = new StringBuilder();
        sb.append("return "); //$NON-NLS-1$
        sb.append(field.getName());
        sb.append(';');
        method.addBodyLine(sb.toString());
        return method;
    }
}

另外， Setter方法的实现，可以参考AbstractJavaGenerator抽象类的getJavaBeansSetter方法，如：

  public Method getJavaBeansSetter(IntrospectedColumn introspectedColumn) {
        FullyQualifiedJavaType fqjt = introspectedColumn
                .getFullyQualifiedJavaType();
        String property = introspectedColumn.getJavaProperty();

        Method method = new Method();
        method.setVisibility(JavaVisibility.PUBLIC);
        method.setName(getSetterMethodName(property));
        method.addParameter(new Parameter(fqjt, property));
        context.getCommentGenerator().addSetterComment(method,
                introspectedTable, introspectedColumn);

        StringBuilder sb = new StringBuilder();
        if (isTrimStringsEnabled() && introspectedColumn.isStringColumn()) {
            sb.append("this."); //$NON-NLS-1$
            sb.append(property);
            sb.append(" = "); //$NON-NLS-1$
            sb.append(property);
            sb.append(" == null ? null : "); //$NON-NLS-1$
            sb.append(property);
            sb.append(".trim();"); //$NON-NLS-1$
            method.addBodyLine(sb.toString());
        } else {
            sb.append("this."); //$NON-NLS-1$
            sb.append(property);
            sb.append(" = "); //$NON-NLS-1$
            sb.append(property);
            sb.append(';');
            method.addBodyLine(sb.toString());
        }

        return method;
    }

然后，重写modelExampleClassGenerated产生的方法，如：

  @Override
  public boolean modelExampleClassGenerated(TopLevelClass topLevelClass,
      IntrospectedTable introspectedTable) {
    addLimit(topLevelClass, introspectedTable, "limitStart");
    addLimit(topLevelClass, introspectedTable, "limitSize");
    return super.modelExampleClassGenerated(topLevelClass,
        introspectedTable);
  }

这样，Example改变就完成了。

XML文件支持分页

接下来，我们需要对产生的XML的selectByExample和selectByExampleWithBLOBs方法添加limitStart和limitSize属性。

为selectByExample添加limitStart和limitSize

  /**
   * 为selectByExample添加limitStart和limitSize
   */
  @Override
  public boolean sqlMapSelectByExampleWithoutBLOBsElementGenerated(
      XmlElement element, IntrospectedTable introspectedTable) {
    XmlElement isNotNullElement = new XmlElement("if");
    isNotNullElement.addAttribute(new Attribute("test",
        "limitStart != null and limitSize >= 0"));
    isNotNullElement.addElement(new TextElement(
        "limit #{limitStart} , #{limitSize}"));
    element.addElement(isNotNullElement);
    return super.sqlMapSelectByExampleWithoutBLOBsElementGenerated(element,
        introspectedTable);
  }

为selectByExampleWithBLOBs添加limitStart和limitSize

  /**
   * 为selectByExampleWithBLOBs添加limitStart和limitSize
   */
  @Override
  public boolean sqlMapSelectByExampleWithBLOBsElementGenerated(
      XmlElement element, IntrospectedTable introspectedTable) {
    XmlElement isNotNullElement = new XmlElement("if");
    isNotNullElement.addAttribute(new Attribute("test",
        "limitStart != null and limitSize >= 0"));
    isNotNullElement.addElement(new TextElement(
        "limit #{limitStart} , #{limitSize}"));
    element.addElement(isNotNullElement);
    return super.sqlMapSelectByExampleWithBLOBsElementGenerated(element,
        introspectedTable);
  }

MysqlPaginationPlugin类完整代码

package my.mabatis.example.plugin;

import java.util.List;

import org.mybatis.generator.api.CommentGenerator;
import org.mybatis.generator.api.IntrospectedTable;
import org.mybatis.generator.api.PluginAdapter;
import org.mybatis.generator.api.dom.java.Field;
import org.mybatis.generator.api.dom.java.JavaVisibility;
import org.mybatis.generator.api.dom.java.Method;
import org.mybatis.generator.api.dom.java.Parameter;
import org.mybatis.generator.api.dom.java.PrimitiveTypeWrapper;
import org.mybatis.generator.api.dom.java.TopLevelClass;
import org.mybatis.generator.api.dom.xml.Attribute;
import org.mybatis.generator.api.dom.xml.TextElement;
import org.mybatis.generator.api.dom.xml.XmlElement;
import org.mybatis.generator.codegen.AbstractJavaGenerator;

/**
 * MyBatis MySQL自动生成带分页插件
 * 
 * @author wangmengjun
 *
 */
public class MysqlPaginationPlugin extends PluginAdapter {

  @Override
  public boolean modelExampleClassGenerated(TopLevelClass topLevelClass,
      IntrospectedTable introspectedTable) {
    addLimit(topLevelClass, introspectedTable, "limitStart");
    addLimit(topLevelClass, introspectedTable, "limitSize");
    return super.modelExampleClassGenerated(topLevelClass,
        introspectedTable);
  }

  /**
   * 为selectByExample添加limitStart和limitSize
   */
  @Override
  public boolean sqlMapSelectByExampleWithoutBLOBsElementGenerated(
      XmlElement element, IntrospectedTable introspectedTable) {
    XmlElement isNotNullElement = new XmlElement("if");
    isNotNullElement.addAttribute(new Attribute("test",
        "limitStart != null and limitSize >= 0"));
    isNotNullElement.addElement(new TextElement(
        "limit #{limitStart} , #{limitSize}"));
    element.addElement(isNotNullElement);
    return super.sqlMapSelectByExampleWithoutBLOBsElementGenerated(element,
        introspectedTable);
  }

  /**
   * 为selectByExampleWithBLOBs添加limitStart和limitSize
   */
  @Override
  public boolean sqlMapSelectByExampleWithBLOBsElementGenerated(
      XmlElement element, IntrospectedTable introspectedTable) {
    XmlElement isNotNullElement = new XmlElement("if");
    isNotNullElement.addAttribute(new Attribute("test",
        "limitStart != null and limitSize >= 0"));
    isNotNullElement.addElement(new TextElement(
        "limit #{limitStart} , #{limitSize}"));
    element.addElement(isNotNullElement);
    return super.sqlMapSelectByExampleWithBLOBsElementGenerated(element,
        introspectedTable);
  }

  private void addLimit(TopLevelClass topLevelClass,
      IntrospectedTable introspectedTable, String name) {

    CommentGenerator commentGenerator = context.getCommentGenerator();

    /**
     * 创建类成员变量 如protected Integer limitStart;
     */
    Field field = new Field();
    field.setVisibility(JavaVisibility.PROTECTED);
    field.setType(PrimitiveTypeWrapper.getIntegerInstance());
    field.setName(name);
    commentGenerator.addFieldComment(field, introspectedTable);
    topLevelClass.addField(field);
    /**
     * 首字母大写
     */
    char c = name.charAt(0);
    String camel = Character.toUpperCase(c) + name.substring(1);

    /**
     * 添加Setter方法
     */
    Method method = new Method();
    method.setVisibility(JavaVisibility.PUBLIC);
    method.setName("set" + camel);
    method.addParameter(new Parameter(PrimitiveTypeWrapper
        .getIntegerInstance(), name));

    StringBuilder sb = new StringBuilder();
    sb.append("this.");
    sb.append(name);
    sb.append(" = ");
    sb.append(name);
    sb.append(";");
    /**
     * 如 this.limitStart = limitStart;
     */
    method.addBodyLine(sb.toString());

    commentGenerator.addGeneralMethodComment(method, introspectedTable);
    topLevelClass.addMethod(method);

    /**
     * 添加Getter Method 直接调用AbstractJavaGenerator的getGetter方法
     */
    Method getterMethod = AbstractJavaGenerator.getGetter(field);
    commentGenerator.addGeneralMethodComment(getterMethod,
        introspectedTable);
    topLevelClass.addMethod(getterMethod);
  }

  public boolean validate(List<String> warnings) {
    return true;
  }

}

修改自动产生代码配置文件generatorConfig.xml中的plugin。

    <!-- 配置内置的或者自定义的Plugin -->
    <plugin type="my.mabatis.example.plugin.MysqlPaginationPlugin" />

自动产生代码，我们可以看到NewsExample.java以及NewsMapper.xml都具有limitStart和limitSize, 可以支持分页。部分相关代码如下：

package my.mybatis.generator.auto.entity;

import java.util.ArrayList;
import java.util.Date;
import java.util.List;

public class NewsExample {

    /**
     * This field was generated by MyBatis Generator.
     * This field corresponds to the database table m_news
     *
     * @mbggenerated Wed Nov 09 21:39:59 CST 2016
     */
    protected Integer limitStart;

    /**
     * This field was generated by MyBatis Generator.
     * This field corresponds to the database table m_news
     *
     * @mbggenerated Wed Nov 09 21:39:59 CST 2016
     */
    protected Integer limitSize;


    /**
     * This method was generated by MyBatis Generator.
     * This method corresponds to the database table m_news
     *
     * @mbggenerated Wed Nov 09 21:39:59 CST 2016
     */
    public void setLimitStart(Integer limitStart) {
        this.limitStart = limitStart;
    }

    /**
     * This method was generated by MyBatis Generator.
     * This method corresponds to the database table m_news
     *
     * @mbggenerated Wed Nov 09 21:39:59 CST 2016
     */
    public Integer getLimitStart() {
        return limitStart;
    }

    /**
     * This method was generated by MyBatis Generator.
     * This method corresponds to the database table m_news
     *
     * @mbggenerated Wed Nov 09 21:39:59 CST 2016
     */
    public void setLimitSize(Integer limitSize) {
        this.limitSize = limitSize;
    }

    /**
     * This method was generated by MyBatis Generator.
     * This method corresponds to the database table m_news
     *
     * @mbggenerated Wed Nov 09 21:39:59 CST 2016
     */
    public Integer getLimitSize() {
        return limitSize;
    }
    //省略其它
}

 <select id="selectByExampleWithBLOBs" parameterType="my.mybatis.generator.auto.entity.NewsExample" resultMap="ResultMapWithBLOBs">
    <!--
      WARNING - @mbggenerated
      This element is automatically generated by MyBatis Generator, do not modify.
      This element was generated on Wed Nov 09 21:39:59 CST 2016.
    -->
    select
    <if test="distinct">
      distinct
    </if>
    <include refid="Base_Column_List" />
    ,
    <include refid="Blob_Column_List" />
    from m_news
    <if test="_parameter != null">
      <include refid="Example_Where_Clause" />
    </if>
    <if test="orderByClause != null">
      order by ${orderByClause}
    </if>
    <if test="limitStart != null and limitSize &gt;= 0">
      limit #{limitStart} , #{limitSize}
    </if>
  </select>
  <select id="selectByExample" parameterType="my.mybatis.generator.auto.entity.NewsExample" resultMap="BaseResultMap">
    <!--
      WARNING - @mbggenerated
      This element is automatically generated by MyBatis Generator, do not modify.
      This element was generated on Wed Nov 09 21:39:59 CST 2016.
    -->
    select
    <if test="distinct">
      distinct
    </if>
    <include refid="Base_Column_List" />
    from m_news
    <if test="_parameter != null">
      <include refid="Example_Where_Clause" />
    </if>
    <if test="orderByClause != null">
      order by ${orderByClause}
    </if>
    <if test="limitStart != null and limitSize &gt;= 0">
      limit #{limitStart} , #{limitSize}
    </if>

至此，大功告成。

▼

测试

创建一个用于获取分页列表的方法。

package my.mabatis.example.service;

import java.util.List;

import my.mabatis.example.util.MyBatisUtil;
import my.mybatis.generator.auto.dao.UserMapper;
import my.mybatis.generator.auto.entity.User;
import my.mybatis.generator.auto.entity.UserExample;
import my.mybatis.generator.auto.entity.UserExample.Criteria;

import org.apache.ibatis.session.SqlSession;

/**
 * 
 * @author wangmengjun
 *
 */
public class UserService {

  /**
   * 查找分页列表
   */
  public List<User> selectNewsByPage(int pageNo, int pageSize) {

    SqlSession sqlSession = MyBatisUtil.getSqlSessionFactory()
        .openSession();
    try {
      UserMapper userDao = sqlSession.getMapper(UserMapper.class);
      /**
       * 使用Example来操作
       */
      UserExample example = new UserExample();
      /**
       * 设置limitStart和limitSize
       */
      example.setLimitStart((pageNo - 1) * pageSize);
      example.setLimitSize(pageSize);
      return userDao.selectByExample(example);
    } finally {
      sqlSession.close();
    }
  }

}

写一个测试类，获取第一页数据，一页5条

package my.mabatis.example.runner;

import java.util.List;

import my.mabatis.example.service.UserService;
import my.mybatis.generator.auto.entity.User;

public class Test {

  public static void main(String[] args) {
    UserService userService = new UserService();
    /**
     * 获取第一页的数据, 一页5条数据
     */
    List<User> users = userService.selectNewsByPage(1, 5);
    System.out.println(users.size());
  }
}

测试数据一共有三条，所以返回结果是正确的。

log4j:WARN No appenders could be found for logger (org.apache.ibatis.logging.LogFactory).
log4j:WARN Please initialize the log4j system properly.
log4j:WARN See http://logging.apache.org/log4j/1.2/faq.html#noconfig for more info.