Syntax of #{} and ${}

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1 Introduction

How to use #{} and ${} and the difference between them are extended by the following code.

//数据库中的语法
INSERT INTO account (aname,alias_name,age) VALUES('玛莎','莎莎',20);
//jdbc中为了防止SQL注入我们使用prepareStatement 
//这种从对象中获取属性值的方式，是通过OGNL对象图表达式语言完成的。
Account account = new Account();
account.setAname("张三")
account.setAlias_name("三")
account.setAge(20)
PreparedStatement pre = connection.prepareStatement("insert into account (aname,alias_name,age) values(?,?,?)");
pre.setString(1,account.getAname());
pre.setString(2,account.getAlias_name());
pe.setString(3,account.getAge());

2. The difference between #{} and ${}

Similarities: all are parameter substitution
Difference 1: #() will be automatically added according to the parameter type'' and $() is just a replacement
Difference 2: #()In the conditional statement where aid=#(aid) can directly use #(), if it is $(), an error will be reported.
Here is the specific code demonstration:

   <select id="selectOne" resultType="Account">
        select
        aid,aname,alias_name as aliasname,age
        from
        account
        where aid=${aid}
    </select>

Error message:

Cause: org.apache.ibatis.reflection.ReflectionException: There is no getter for property named ‘aid’ in ‘class java.lang.Integer’

Solution:
${} For int and Integer numeric types, it must be passed ${value}.
Once identified, ${value}it will be replaced directly, without using OGNL

Why do you say but ${}just replace what does it mean?
AccountMapper.xml file

    <insert id="insert">
        insert
        into
        account(aname,alias_name,age)
        values (${aname},${aliasname},${age})
     </insert>

test:

@Test
 public void insert(){
    
    
     account.setAge(25);
     account.setAliasname("智智");
     account.setAname("李智234789");
     accountMapper.insertAccount(account);
 }

Result: After
running, we found that the SQL statement values value did not have double quotes
SQL: insert into account(aname,alias_name,age) values (李智234,智智,25)
So the database does not recognize this statement but we can also add it manually ''
For example, the following operations:

 @Test
    public void insert(){
    
    
        account.setAge(25);
        account.setAliasname("'智智'");
        account.setAname("'李智234789'");
        accountMapper.insertAccount(account);
    }

After running, we found that this statement
was added to the database. Obviously it is very cumbersome, so it will appear #{} He will help us to automatically add according to the type of attribute. ''
Modify the above configuration file AccountMapper.xml or the values statement asvalues ('${aname}','${aliasname}','${age}')
实质上:就是利用 prepareStatement进行sql的执行

  <insert id="insert">
        insert
        into
        account(aname,alias_name,age)
        values (#{
    
    aname},#{
    
    aliasname},#{
    
    age})
  </insert>

3. Mohu query

#{}模胡Query
AccountMapper.xml file

  <select id="selectLike" resultType="account">
    select
     aid,aname,alias_name as aliasname,age
    from
     account
     where aname like #{
    
    anme}
  </select>

test:

 @Test
    public void select(){
    
    
        List<Account> list = accountMapper.selectLike("%李%");
        System.out.println(JSON.toJSONString(list));
    }

result:

[{"Age":23,"aid":19,"aliasname":"瑞瑞","aname":"李瑞"},{"age":20,"aid":21,"aliasname": "智智","aname":"李智"}]

Summary: Use #() fuzzy query to pass value usage%李%

#{}Fuzzy query

 <select id="selectLike" resultType="account">
    select
     aid,aname,alias_name as aliasname,age
    from
     account
     <!-- 
     where aname like '${value}'
      List<Account> list = accountMapper.selectLike("%李%");
     -->
     where aname like ${value}
  </select>

@Test
    public void select(){
    
    
        List<Account> list = accountMapper.selectLike("'%李%'");
        System.out.println(JSON.toJSONString(list));
    }