jpa使用entityManager直接sql语句查询

前言

最近在使用jpa做新项目，其中涉及到一些动态的数据结构（构建不出model类的）需要查询，因此调研了一下jpa怎么直接使用entityManager跑sql

获取entityManager

正常在spring boot中配置数据库连接之后，直接在需要用的类中加入

    @Autowired
    private EntityManager entityManager;

如果配置了多数据源需要指定注入（网上关于多数据源找到entityManager的教程不少）

获取query对象

看一下EntityManager的底层源码中关于NativeQuery的部分

    /**
     * Create an instance of <code>Query</code> for executing
     * a native SQL statement, e.g., for update or delete.
     * If the query is not an update or delete query, query
     * execution will result in each row of the SQL result
     * being returned as a result of type Object[] (or a result
     * of type Object if there is only one column in the select
     * list.)  Column values are returned in the order of their
     * appearance in the select list and default JDBC type
     * mappings are applied.
     * @param sqlString a native SQL query string
     * @return the new query instance
     */
    public Query createNativeQuery(String sqlString);

    /**
     * Create an instance of <code>Query</code> for executing
     * a native SQL query.
     * @param sqlString a native SQL query string
     * @param resultClass the class of the resulting instance(s)
     * @return the new query instance
     */
    public Query createNativeQuery(String sqlString, Class resultClass);

    /**
     * Create an instance of <code>Query</code> for executing
     * a native SQL query.
     * @param sqlString a native SQL query string
     * @param resultSetMapping the name of the result set mapping
     * @return the new query instance
     */
    public Query createNativeQuery(String sqlString, String resultSetMapping);

第一个很简单，直接输入一个sql语句字符串即可。
第二个是定义一个返回的类结构
第三个是对结果集进行自定义映射

获取查询结果

获取到query对象之后看一下Query类的底层代码

    /**
     * Execute a SELECT query and return the query results
     * as an untyped List.
     * @return a list of the results
     * @throws IllegalStateException if called for a Java
     *         Persistence query language UPDATE or DELETE statement
     * @throws QueryTimeoutException if the query execution exceeds
     *         the query timeout value set and only the statement is
     *         rolled back
     * @throws TransactionRequiredException if a lock mode other than
     *         <code>NONE</code> has been set and there is no transaction
     *         or the persistence context has not been joined to the transaction
     * @throws PessimisticLockException if pessimistic locking
     *         fails and the transaction is rolled back
     * @throws LockTimeoutException if pessimistic locking
     *         fails and only the statement is rolled back
     * @throws PersistenceException if the query execution exceeds 
     *         the query timeout value set and the transaction 
     *         is rolled back 
     */
    List getResultList();

    /**
     * Execute a SELECT query and return the query results
     * as an untyped <code>java.util.stream.Stream</code>.
     * By default this method delegates to <code>getResultList().stream()</code>,
     * however persistence provider may choose to override this method
     * to provide additional capabilities.
     *
     * @return a stream of the results
     * @throws IllegalStateException if called for a Java
     *         Persistence query language UPDATE or DELETE statement
     * @throws QueryTimeoutException if the query execution exceeds
     *         the query timeout value set and only the statement is
     *         rolled back
     * @throws TransactionRequiredException if a lock mode other than
     *         <code>NONE</code> has been set and there is no transaction
     *         or the persistence context has not been joined to the transaction
     * @throws PessimisticLockException if pessimistic locking
     *         fails and the transaction is rolled back
     * @throws LockTimeoutException if pessimistic locking
     *         fails and only the statement is rolled back
     * @throws PersistenceException if the query execution exceeds
     *         the query timeout value set and the transaction
     *         is rolled back
     * @see Stream
     * @see #getResultList()
     * @since 2.2
     */
    default Stream getResultStream() {
        return getResultList().stream();
    }

第一个是直接把结果当作List返回，第二个是将结果当作stream流返回。