java - 处理 MySQL 查询花费的时间太长并抛出 "Communications link failure"

Question

我使用 JDBC 连接发出查询:

Connection conn = null
Class.forName("com.mysql.jdbc.Driver")
conn = DriverManager.getConnection(dbHost, dbUser, dbPass)

s = conn.createStatement(java.sql.ResultSet.TYPE_FORWARD_ONLY, java.sql.ResultSet.CONCUR_READ_ONLY);
s.setFetchSize(Integer.MIN_VALUE);
r = s.executeQuery (MY_SELECT_QUERY);
while(...) { 
  processResultSet(r);
}

while 长时间运行处理 ResultSet。大约 1 小时后，我得到了异常:

com.mysql.jdbc.exceptions.jdbc4.CommunicationsException: Communications link failure

The last packet successfully received from the server was 74 milliseconds ago. The last packet sent successfully to the server was 4,351,980 milliseconds ago.

   at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method) 
   at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:39)

   at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:27)

   at java.lang.reflect.Constructor.newInstance(Constructor.java:513) 
   at com.mysql.jdbc.Util.handleNewInstance(Util.java:407) 
   at com.mysql.jdbc.SQLError.createCommunicationsException(SQLError.java:1116)

   at com.mysql.jdbc.MysqlIO.reuseAndReadPacket(MysqlIO.java:3082) 
   at com.mysql.jdbc.MysqlIO.reuseAndReadPacket(MysqlIO.java:2968) 
   at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:3516) 
   at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:931) 
   at com.mysql.jdbc.MysqlIO.nextRow(MysqlIO.java:1429) 
   at com.mysql.jdbc.RowDataDynamic.nextRecord(RowDataDynamic.java:416) 
   at com.mysql.jdbc.RowDataDynamic.next(RowDataDynamic.java:395) 
   at com.mysql.jdbc.ResultSetImpl.next(ResultSetImpl.java:7171) 
   at com.busk.indexer.AbstractIndexerClient.indexResultSet(AbstractIndexerClient.scala:34)

   at com.busk.indexer.ContinousIndexer.startIndexingLoop(ContinousIndexer.scala:18)

   at com.busk.indexer.IndexerServer$.main(IndexerServer.scala:95) 
   at com.busk.indexer.IndexerServer.main(IndexerServer.scala) 
   at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) 
   at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)

   at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)

   at java.lang.reflect.Method.invoke(Method.java:597)

我用谷歌搜索了一下，但几乎所有错误都与连接池有关，并且使用一个连接超过 8 小时(默认的 wait_timeout mysql 设置)。在我的例子中，我正在处理 ResultSet，我收到了这个错误。

Answer 1

如果发生这种情况是因为结果集中有大量数据，我建议使用“LIMIT s, m”(其中 s - 开始，m - 最大记录 - 均为整数)对响应进行分页。然后在循环中处理此数据，一次处理 1000 条记录的一部分:

boolean finished = false;
int start = 1;
int max = 1000;
do {
   r = s.executeQuery (MY_SELECT_QUERY + " LIMIT " + start + ", " + max);
   finished = // if r returned less than M records
   while(...) { 
     processResultSet(r);
   } 
   start += // number of returned records;
} while (!finished);

当然，1000 是一个任意数字，您可以尝试找出适合您查询的最佳最大大小。

更新:如果处理每条记录需要很长时间，则将记录存储在一个列表中，并在从数据库中获取所有数据时遍历它们。