我正在尝试制作一个幻像读物，为了学习，但不幸的是我无法做到。我正在使用Java线程，JDBC，MySQL。 这是我正在使用的程序：
package com.isolation.levels.phenomensa;
import javax.xml.transform.Result;
import java.sql.Connection;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.util.concurrent.CountDownLatch;
import static com.isolation.levels.ConnectionsProvider.getConnection;
import static com.isolation.levels.Utils.printResultSet;
/**
* Created by dreambig on 13.03.17.
*/
public class PhantomReads {
public static void main(String[] args) {
setUp(getConnection());// delete the newly inserted row, the is supposed to be a phantom row
CountDownLatch countDownLatch1 = new CountDownLatch(1); // use to synchronize threads steps
CountDownLatch countDownLatch2 = new CountDownLatch(1); // use to synchronize threads steps
Transaction1 transaction1 = new Transaction1(countDownLatch1, countDownLatch2, getConnection()); // the first runnable
Transaction2 transaction2 = new Transaction2(countDownLatch1, countDownLatch2, getConnection()); // the second runnable
Thread thread1 = new Thread(transaction1); // transaction 1
Thread thread2 = new Thread(transaction2); // transaction 2
thread1.start();
thread2.start();
}
private static void setUp(Connection connection) {
try {
connection.prepareStatement("DELETE from actor where last_name=\"PHANTOM_READ\"").execute();
} catch (SQLException e) {
e.printStackTrace();
}
}
public static class Transaction1 implements Runnable {
private CountDownLatch countDownLatch;
private CountDownLatch countDownLatch2;
private Connection connection;
public Transaction1(CountDownLatch countDownLatch, CountDownLatch countDownLatch2, Connection connection) {
this.countDownLatch = countDownLatch;
this.countDownLatch2 = countDownLatch2;
this.connection = connection;
}
@Override
public void run() {
try {
String query = "select * from actor where first_name=\"BELA\"";
connection.setAutoCommit(false); // start the transaction
// the transaction isolation, dirty reads and non-repeatable reads are prevented !
// only phantom reads can occure
connection.setTransactionIsolation(Connection.TRANSACTION_REPEATABLE_READ);
//read the query result for the first time.
ResultSet resultSet = connection.prepareStatement(query).executeQuery();
printResultSet(resultSet); // print result.
//count down so that thread2 can insert a row and commit.
countDownLatch2.countDown();
//wait for the second query the finish inserting the row
countDownLatch.await();
System.out.println("\n ********* The query returns a second row satisfies it (a phantom read) ********* !");
//query the result again ...
ResultSet secondRead = connection.createStatement().executeQuery(query);
printResultSet(secondRead); //print the result
} catch (SQLException e) {
e.printStackTrace();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
public static class Transaction2 implements Runnable {
private CountDownLatch countDownLatch;
private CountDownLatch countDownLatch2;
private Connection connection;
public Transaction2(CountDownLatch countDownLatch, CountDownLatch countDownLatch2, Connection connection) {
this.countDownLatch = countDownLatch;
this.countDownLatch2 = countDownLatch2;
this.connection = connection;
}
@Override
public void run() {
try {
//wait the first thread to read the result
countDownLatch2.await();
//insert and commit !
connection.prepareStatement("INSERT INTO actor (first_name,last_name) VALUE (\"BELA\",\"PHANTOM_READ\") ").execute();
//count down so that the thread1 can read the result again ...
countDownLatch.countDown();
} catch (SQLException e) {
e.printStackTrace();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}
然而，这实际上是结果
----------------------------------------------------------
| 196 | | BELA | | WALKEN | | 2006-02-15 04:34:33.0 |
---------------------------------------------------------- The query returns a second row satisfies it (a phantom read)
----------------------------------------------------------
| 196 | | BELA | | WALKEN | | 2006-02-15 04:34:33.0 |
----------------------------------------------------------
但我认为应该是
----------------------------------------------------------
| 196 | | BELA | | WALKEN | | 2006-02-15 04:34:33.0 |
---------------------------------------------------------- The query returns a second row satisfies it (a phantom read) !
----------------------------------------------------------
| 196 | | BELA | | WALKEN | | 2006-02-15 04:34:33.0 |
----------------------------------------------------------
----------------------------------------------------------
| 196 | | BELA | | PHANTOM_READ | | 2006-02-15 04:34:33.0 |
----------------------------------------------------------
我正在使用： Java 8 JDBC MySQL的 InnoDB的 Sakila DB插入mysql
答案 0 :(得分：2)
幻像读取是以下场景：事务读取满足搜索条件的一组行。然后第二个事务插入满足此搜索条件的行。然后第一个事务再次读取满足搜索条件的行集，并获取一组不同的行（例如，包括新插入的行）。
可重复读取要求如果事务读取一行，则另一个事务会更新或删除此行并提交这些更改，并且第一个事务重新读取该行，它将获得相同的常量值和以前一样（快照）。
实际上并不要求幻像读取必须发生。 MySQL实际上会在更多情况下阻止幻像读取。在MySQL中，幻像读取（当前）仅在您（意外地）更新幻像行之后发生，否则该行保持隐藏状态。这是针对MySQL的，其他数据库系统的行为会有所不同。此外，这种行为可能会在某一天发生变化（因为MySQL仅指定它支持sql标准所要求的一致读取，而不是在哪些特定情况下发生幻像读取）。
您可以使用以下步骤来获取幻像行：
insert into actor (first_name,last_name) values ('ADELIN','NO_PHANTOM');
transaction 1:
select * from actor;
-- ADELIN|NO_PHANTOM
transaction 2:
insert into actor (first_name,last_name) values ('BELA','PHANTOM_READ');
commit;
transaction 1:
select * from actor; -- still the same
-- ADELIN|NO_PHANTOM
update actor set last_name = 'PHANTOM READ'
where last_name = 'PHANTOM_READ';
select * from actor; -- now includes the new, updated row
-- ADELIN|NO_PHANTOM
-- BELA |PHANTOM READ
当您删除行时，btw会发生另一件有趣的事情：
insert into actor (first_name,last_name) values ('ADELIN','NO_PHANTOM');
insert into actor (first_name,last_name) values ('BELA','REPEATABLE_READ');
transaction 1:
select * from actor;
-- ADELIN|NO_PHANTOM
-- BELA |REPEATABLE_READ
transaction 2:
delete from actor where last_name = 'REPEATABLE_READ';
commit;
transaction 1:
select * from actor; -- still the same
-- ADELIN|NO_PHANTOM
-- BELA |REPEATABLE_READ
update actor set last_name = '';
select * from actor; -- the deleted row stays unchanged
-- ADELIN|
-- BELA |REPEATABLE_READ
这正是sql标准所要求的：如果你重读（删除）行，你将获得原始值。