weblogic日志中报：java.sql.SQLException:ORA-24778:无法打开连接。分析功能是有多次对远程表的操作,先查看关于db_link的参数：

SQL> show parameter open_links
NAME                                 TYPE        VALUE
------------------------------------ ----------- -------
open_links                           integer     4
open_links_per_instance              integer     4

修改参数后重启系统：

alter system set open_links=50 scope=spfile;
alter system set open_links_per_instance=255 scope=spfile;

官方文档大致的意思是：

open_links是单个会话打开远程连接的最大数目，包括外部的存储过程，外部的进程（反正就是外部的东西）。默认值是4，最大值是255，当设置为0的时候说明没有分布式事务。

open_links_per_instance是一个实例打开远程连接的最大数目。默认值是4，最大是4294967295，运用于JDBC的XA驱动，而open_links不运用于JDBC的XA驱动。

我认为open_links_per_instance大小设置应该是open_links的几倍，且在刚建立数据库时就应该明确，后期修改重启有点郁闷。

OPEN_LINKS

OPEN_LINKS specifies the maximum number of concurrent open connections to remote databases in one session. These connections include database links, as well as external procedures and cartridges, each of which uses a separate process.

Oracle counts one open link for the following:

• For each user that references a public or private database link

• For each external procedure or cartridge connection when it is executed for the first time

Both types of connections close when the session ends. You can also close a database link connection explicitly by issuing an ALTER SESSION CLOSE DATABASE LINK statement.

You should set this parameter to allow for the external procedure and cartridge connections expected during the session plus the number of databases referred to in typical distributed transactions (that is, a single SQL statement that references multiple databases), so that all the databases can be open to execute the statement. For example, if queries alternately access databases A, B, and C, and OPEN_LINKS is set to 2, time will be lost waiting while one connection is broken and another made. Increase the value if many different databases are accessed over time.

This parameter refers only to connections used for distributed transactions. Direct connections to a remote database specified as an application connects are not counted.

If you set OPEN_LINKS to 0, then no distributed transactions are allowed.

OPEN_LINKS_PER_INSTANCE

OPEN_LINKS_PER_INSTANCE specifies the maximum number of migratable open connections globally for each database instance. XA transactions use migratable open connections so that the connections are cached after a transaction is committed. Another transaction can use the connection, provided the user who created the connection is the same as the user who owns the transaction.

OPEN_LINKS_PER_INSTANCE is different from OPEN_LINKS, which indicates the number of connections from a session. The OPEN_LINKS parameter is not applicable to XA applications.

java.sql.SQLException:ORA-24778:无法打开连接