在JDK源代码java.sql.Connection类中，明确定义了JDBC支持的4个类型的事务隔离级别，在Connection类的开头部分，就定义了如下5个常量，TRANSACTION_NONE常量只是用来指示不支持事务。
/**
* A constant indicating that transactions are not supported.
*/
int TRANSACTION_NONE = 0;
/**
* A constant indicating that
* dirty reads, non-repeatable reads and phantom reads can occur.
* This level allows a row changed by one transaction to be read
* by another transaction before any changes in that row have been
* committed (a "dirty read"). If any of the changes are rolled back,
* the second transaction will have retrieved an invalid row.
*/
int TRANSACTION_READ_UNCOMMITTED = 1;
/**
* A constant indicating that
* dirty reads are prevented; non-repeatable reads and phantom
* reads can occur. This level only prohibits a transaction
* from reading a row with uncommitted changes in it.
*/
int TRANSACTION_READ_COMMITTED = 2;
/**
* A constant indicating that
* dirty reads and non-repeatable reads are prevented; phantom
* reads can occur. This level prohibits a transaction from
* reading a row with uncommitted changes in it, and it also
* prohibits the situation where one transaction reads a row,
* a second transaction alters the row, and the first transaction
* rereads the row, getting different values the second time
* (a "non-repeatable read").
*/
int TRANSACTION_REPEATABLE_READ = 4;
/**
* A constant indicating that
* dirty reads, non-repeatable reads and phantom reads are prevented.
* This level includes the prohibitions in
* <code>TRANSACTION_REPEATABLE_READ</code> and further prohibits the
* situation where one transaction reads all rows that satisfy
* a <code>WHERE</code> condition, a second transaction inserts a row that
* satisfies that <code>WHERE</code> condition, and the first transaction
* rereads for the same condition, retrieving the additional
* "phantom" row in the second read.
*/
int TRANSACTION_SERIALIZABLE = 8;
| 隔离级别 | 说明 |
|---|---|
| READ_UNCOMMITTED | 一个事务可以读取另外一个事物尚未提交的数据，就是俗称“脏读”（dirty read），在没有提交数据时能够读到已经更新的数据 |
| TRANSACTION_READ_COMMITTED | 在一个事务中进行查询时，允许读取提交前的数据，数据提交后，当前查询就可以读取到数据。update数据时候并不锁住表 |
| TRANSACTION_REPEATABLE_READ | 在一个事务中进行查询时，不允许读取其他事务update的数据，允许读取到其他事务提交的新增数据 |
| TRANSACTION_SERIALIZABLE | 在一个事务中进行查询时，不允许任何对这个查询表的数据修改 |
在Spring中设置食物隔离级别对应的接口：
/**
* Attempts to change the transaction isolation level for this
* <code>Connection</code> object to the one given.
* The constants defined in the interface <code>Connection</code>
* are the possible transaction isolation levels.
* <P>
* <B>Note:</B> If this method is called during a transaction, the result
* is implementation-defined.
*
* @param level one of the following <code>Connection</code> constants:
* <code>Connection.TRANSACTION_READ_UNCOMMITTED</code>,
* <code>Connection.TRANSACTION_READ_COMMITTED</code>,
* <code>Connection.TRANSACTION_REPEATABLE_READ</code>, or
* <code>Connection.TRANSACTION_SERIALIZABLE</code>.
* (Note that <code>Connection.TRANSACTION_NONE</code> cannot be used
* because it specifies that transactions are not supported.)
* @exception SQLException if a database access error occurs, this
* method is called on a closed connection
* or the given parameter is not one of the <code>Connection</code>
* constants
* @see DatabaseMetaData#supportsTransactionIsolationLevel
* @see #getTransactionIsolation
*/
void setTransactionIsolation(int level) throws SQLException;
