求助:MYSQL出错:java.sql.SQLException: No operations allowed after connection closed

web应用平台是apache2.51 + tomcat4.1.30 ,采用mysql数据库,都在一台机器上,在并发100以下用户时不会出现任何问题,但在做5次100并发测试时,tomcat控制台报错java.sql.SQLException: No operations allowed after connection closed进入mysql时发现
Max_used_connections     | 90 
而mysql的最大连接数据是500,远远高于90,
show processlist;也只有50左右
数据库的连接池的大小是400,show status;状态如下:
| Max_used_connections     | 90         |
| Not_flushed_key_blocks   | 0          |
| Not_flushed_delayed_rows | 0          |
| Open_tables              | 142        |
| Open_files               | 170        |
| Open_streams             | 0          |
| Opened_tables            | 148        |
| Questions                | 212930     |
| Qcache_queries_in_cache  | 1083       |
| Qcache_inserts           | 31126      |
| Qcache_hits              | 135176     |
| Qcache_lowmem_prunes     | 0          |
| Qcache_not_cached        | 1391       |
| Qcache_free_memory       | 15118992   |
| Qcache_free_blocks       | 319        |
| Qcache_total_blocks      | 2523       |
| Rpl_status               | NULL       |
| Select_full_join         | 33886      |
| Select_full_range_join   | 0          |
| Select_range             | 13714      |
| Select_range_check       | 0          |
| Select_scan              | 3236       |
| Slave_open_temp_tables   | 0          |
| Slave_running            | OFF        |
| Slow_launch_threads      | 0          |
| Slow_queries             | 1          |
| Sort_merge_passes        | 0          |
| Sort_range               | 9654       |
| Sort_rows                | 239147     |
| Sort_scan                | 8136       |
| Table_locks_immediate    | 98411      |
| Table_locks_waited       | 549        |
| Threads_cached           | 2          |
| Threads_created          | 7428       |
| Threads_connected        | 53         |
| Threads_running          | 1          |
| Uptime                   | 4294855340 |                                               谁能告诉我是哪出了问题撒,在此谢过了......