mybatis的${}和#{}

${}

${}在组装sql前将里面的表达式计算出来,举例子:

select * from t_user where count=${list.size}

在创建preparedStatement前就将${list.size}计算出来了,假设计算值为10,则会将sql = 
"select * from t_user where count=10" 放入preparedStatement对象中.

#{}

${}在组装sql后将里面的表达式计算出来,举例子:

select * from t_user where count=${list.size}

在创建preparedStatement前就将#{list.size}替换为'?',假设计算值为10,则会将sql = "select 
* from t_user where count=?"以及参数'10' 放入preparedStatement对象中.
当然了,上面这条写法是错误的,因为在mybatis源码中,有各种各样的ObjectWrapper,它们就
是专门来计算对象的属性值的,比如BeanWrapper用来调用getter方法取出对象的属性
值,MapWrapper用来取出key对应的value值,而在这里我们传入的是一个list,是一个集合,对
应的是CollectionWrapper,这个类不支持集合获取属性,所以并不是网上所说的#{}默认加了
一个引号导致了报错,这个类的源码如下:

package org.apache.ibatis.reflection.wrapper;

import java.util.Collection;
import java.util.List;

import org.apache.ibatis.reflection.MetaObject;
import org.apache.ibatis.reflection.factory.ObjectFactory;
import org.apache.ibatis.reflection.property.PropertyTokenizer;

public class CollectionWrapper implements ObjectWrapper {

  private final Collection<Object> object;

  public CollectionWrapper(MetaObject metaObject, Collection<Object> object) {
    this.object = object;
  }

// 就是调用的这个方法去获取"list.size",结果不让取,我猜原因是作者懒得处理了
// 如果传入的是"list.age",那么我们应该抛出UnsupportedOperationException
// 我觉得传入"list.size"应该处理一下,毕竟所有的集合都有'size'这个属性
  @Override
  public Object get(PropertyTokenizer prop) {
    throw new UnsupportedOperationException();
  }

  @Override
  public void set(PropertyTokenizer prop, Object value) {
    throw new UnsupportedOperationException();
  }

  @Override
  public String findProperty(String name, boolean useCamelCaseMapping) {
    throw new UnsupportedOperationException();
  }

  @Override
  public String[] getGetterNames() {
    throw new UnsupportedOperationException();
  }

  @Override
  public String[] getSetterNames() {
    throw new UnsupportedOperationException();
  }

  @Override
  public Class<?> getSetterType(String name) {
    throw new UnsupportedOperationException();
  }

  @Override
  public Class<?> getGetterType(String name) {
    throw new UnsupportedOperationException();
  }

  @Override
  public boolean hasSetter(String name) {
    throw new UnsupportedOperationException();
  }

  @Override
  public boolean hasGetter(String name) {
    throw new UnsupportedOperationException();
  }

  @Override
  public MetaObject instantiatePropertyValue(String name, PropertyTokenizer prop, ObjectFactory objectFactory) {
    throw new UnsupportedOperationException();
  }

  @Override
  public boolean isCollection() {
    return true;
  }

  @Override
  public void add(Object element) {
    object.add(element);
  }

  @Override
  public <E> void addAll(List<E> element) {
    object.addAll(element);
  }

}