MyBatis之DynamicContext

MyBatis官方文档

• MyBatis官方文档
• ORM框架

动态 SQL

• bind
  bind 元素可以从 OGNL 表达式中创建一个变量并将其绑定到上下文。比如：

<select id="selectBlogsLike" resultType="Blog">
  <bind name="pattern" value="'%' + _parameter.getTitle() + '%'" />
  SELECT * FROM BLOG
  WHERE title LIKE #{pattern}
select>

• 多数据库支持
  一个配置了“_databaseId”变量的 databaseIdProvider 可用于动态代码中，这样就可以根据不同的数据库厂商构建特定的语句。比如下面的例子：

<insert id="insert">
  <selectKey keyProperty="id" resultType="int" order="BEFORE">
    <if test="_databaseId == 'oracle'">
      select seq_users.nextval from dual
    if>
    <if test="_databaseId == 'db2'">
      select nextval for seq_users from sysibm.sysdummy1"
    if>
  selectKey>
  insert into users values (#{id}, #{name})
insert>

源码

package org.apache.ibatis.scripting.xmltags;

import java.util.HashMap;
import java.util.Map;
import ognl.OgnlContext;
import ognl.OgnlException;
import ognl.OgnlRuntime;
import ognl.PropertyAccessor;
import org.apache.ibatis.reflection.MetaObject;
import org.apache.ibatis.session.Configuration;

/**
 * @author Clinton Begin
 */
public class DynamicContext {

  public static final String PARAMETER_OBJECT_KEY = "_parameter";
  public static final String DATABASE_ID_KEY = "_databaseId";

  // 静态代码块
  // 特点：随着类的加载而执行，且只执行一次，并优先于主函数，用于给类初始化的。
  static {
    OgnlRuntime.setPropertyAccessor(ContextMap.class, new ContextAccessor());
  }

  private final ContextMap bindings;
  private final StringBuilder sqlBuilder = new StringBuilder();
  private int uniqueNumber = 0;

  public DynamicContext(Configuration configuration, Object parameterObject) {
    if (parameterObject != null && !(parameterObject instanceof Map)) {
      MetaObject metaObject = configuration.newMetaObject(parameterObject);
      bindings = new ContextMap(metaObject);
    } else {
      bindings = new ContextMap(null);
    }
    bindings.put(PARAMETER_OBJECT_KEY, parameterObject);
    bindings.put(DATABASE_ID_KEY, configuration.getDatabaseId());
  }

  public Map<String, Object> getBindings() {
    return bindings;
  }

  public void bind(String name, Object value) {
    bindings.put(name, value);
  }

  public void appendSql(String sql) {
    sqlBuilder.append(sql);
    sqlBuilder.append(" ");
  }

  public String getSql() {
    return sqlBuilder.toString().trim();
  }

  public int getUniqueNumber() {
    return uniqueNumber++;
  }

  // 静态内部类
  static class ContextMap extends HashMap<String, Object> {
    private static final long serialVersionUID = 2977601501966151582L;

    private MetaObject parameterMetaObject;
    public ContextMap(MetaObject parameterMetaObject) {
      this.parameterMetaObject = parameterMetaObject;
    }

    @Override
    public Object get(Object key) {
      String strKey = (String) key;
      if (super.containsKey(strKey)) {
        return super.get(strKey);
      }

      if (parameterMetaObject != null) {
        // issue #61 do not modify the context when reading
        return parameterMetaObject.getValue(strKey);
      }

      return null;
    }
  }
  
  // 静态内部类
  static class ContextAccessor implements PropertyAccessor {

    @Override
    public Object getProperty(Map context, Object target, Object name)
        throws OgnlException {
      Map map = (Map) target;

      Object result = map.get(name);
      if (map.containsKey(name) || result != null) {
        return result;
      }

      Object parameterObject = map.get(PARAMETER_OBJECT_KEY);
      if (parameterObject instanceof Map) {
        return ((Map)parameterObject).get(name);
      }

      return null;
    }

    @Override
    public void setProperty(Map context, Object target, Object name, Object value)
        throws OgnlException {
      Map<Object, Object> map = (Map<Object, Object>) target;
      map.put(name, value);
    }

    @Override
    public String getSourceAccessor(OgnlContext arg0, Object arg1, Object arg2) {
      return null;
    }

    @Override
    public String getSourceSetter(OgnlContext arg0, Object arg1, Object arg2) {
      return null;
    }
  }
}