Tomcat maxThreads maxConnections acceptCount参数说明

首先看官网对这个三个参数的说明：
http://tomcat.apache.org/tomcat-7.0-doc/config/http.html

Attribute	Description
acceptCount	The maximum queue length for incoming connection requests when all possible request processing threads are in use. Any requests received when the queue is full will be refused. The default value is 100.
maxConnections	The maximum number of connections that the server will accept and process at any given time. When this number has been reached, the server will accept, but not process, one further connection. This additional connection be blocked until the number of connections being processed falls below maxConnections at which point the server will start accepting and processing new connections again. Note that once the limit has been reached, the operating system may still accept connections based on the acceptCount setting. The default value varies by connector type. For BIO the default is the value of maxThreads unless an Executor is used in which case the default will be the value of maxThreads from the executor. For NIO the default is 10000. For APR/native, the default is 8192. Note that for APR/native on Windows, the configured value will be reduced to the highest multiple of 1024 that is less than or equal to maxConnections. This is done for performance reasons. If set to a value of -1, the maxConnections feature is disabled and connections are not counted.
maxThreads	The maximum number of request processing threads to be created by this Connector, which therefore determines the maximum number of simultaneous requests that can be handled. If not specified, this attribute is set to 200. If an executor is associated with this connector, this attribute is ignored as the connector will execute tasks using the executor rather than an internal thread pool. Note that if an executor is configured any value set for this attribute will be recorded correctly but it will be reported (e.g. via JMX) as -1 to make clear that it is not used.

用httpClient作为客户端对不同配置分别发10个请求，服务端死循环让请求一直没有返回，然后查看连接情况。
注意，在用httpClient发送请求的时候一定要设置这个参数，否则默认最多只能发送两个并发请求。
// Increase default max connection per route to 20
cm.setDefaultMaxPerRoute(20);

一##

tomcat配置：
connectionTimeout="20000"
redirectPort="8443" maxConnections="3" maxThreads="2" acceptCount="4" />

连接情况7个为ESTABLISHED，3个为SYN_SENT：

netstat -an | grep 8090##

tcp4 158 0 127.0.0.1.8090 127.0.0.1.61515 ESTABLISHED
tcp4 158 0 127.0.0.1.8090 127.0.0.1.61514 ESTABLISHED
tcp4 158 0 127.0.0.1.8090 127.0.0.1.61513 ESTABLISHED
tcp4 158 0 127.0.0.1.8090 127.0.0.1.61512 ESTABLISHED
tcp4 158 0 127.0.0.1.8090 127.0.0.1.61511 ESTABLISHED
tcp4 0 0 127.0.0.1.8090 127.0.0.1.61510 ESTABLISHED
tcp4 0 0 127.0.0.1.8090 127.0.0.1.61509 ESTABLISHED
tcp4 0 0 127.0.0.1.61510 127.0.0.1.8090 ESTABLISHED
tcp4 0 0 127.0.0.1.61511 127.0.0.1.8090 ESTABLISHED
tcp4 0 0 127.0.0.1.61512 127.0.0.1.8090 ESTABLISHED
tcp4 0 0 127.0.0.1.61513 127.0.0.1.8090 ESTABLISHED
tcp4 0 0 127.0.0.1.61514 127.0.0.1.8090 ESTABLISHED
tcp4 0 0 127.0.0.1.61515 127.0.0.1.8090 ESTABLISHED
tcp4 0 0 127.0.0.1.61517 127.0.0.1.8090 SYN_SENT
tcp4 0 0 127.0.0.1.61509 127.0.0.1.8090 ESTABLISHED
tcp4 0 0 127.0.0.1.61518 127.0.0.1.8090 SYN_SENT
tcp4 0 0 127.0.0.1.61516 127.0.0.1.8090 SYN_SENT
tcp46 0 0 *.8090 . LISTEN

二##

tomcat配置：

连接情况6个为ESTABLISHED，4个为SYN_SENT：

netstat -an | grep 8090#

tcp4 0 0 127.0.0.1.61672 127.0.0.1.8090 SYN_SENT
tcp4 0 0 127.0.0.1.61671 127.0.0.1.8090 SYN_SENT
tcp4 158 0 127.0.0.1.8090 127.0.0.1.61663 ESTABLISHED
tcp4 158 0 127.0.0.1.8090 127.0.0.1.61670 ESTABLISHED
tcp4 158 0 127.0.0.1.8090 127.0.0.1.61669 ESTABLISHED
tcp4 158 0 127.0.0.1.8090 127.0.0.1.61668 ESTABLISHED
tcp4 0 0 127.0.0.1.8090 127.0.0.1.61662 ESTABLISHED
tcp4 0 0 127.0.0.1.8090 127.0.0.1.61661 ESTABLISHED
tcp4 0 0 127.0.0.1.61669 127.0.0.1.8090 ESTABLISHED
tcp4 0 0 127.0.0.1.61668 127.0.0.1.8090 ESTABLISHED
tcp4 0 0 127.0.0.1.61661 127.0.0.1.8090 ESTABLISHED
tcp4 0 0 127.0.0.1.61667 127.0.0.1.8090 SYN_SENT
tcp4 0 0 127.0.0.1.61662 127.0.0.1.8090 ESTABLISHED
tcp4 0 0 127.0.0.1.61663 127.0.0.1.8090 ESTABLISHED
tcp4 0 0 127.0.0.1.61664 127.0.0.1.8090 SYN_SENT
tcp4 0 0 127.0.0.1.61670 127.0.0.1.8090 ESTABLISHED
tcp46 0 0 *.8090 . LISTEN

三##

tomcat配置：

连接情况3个为ESTABLISHED，7个为SYN_SENT：

netstat -an | grep 8090#

tcp4 158 0 127.0.0.1.8090 127.0.0.1.61748 ESTABLISHED
tcp4 0 0 127.0.0.1.8090 127.0.0.1.61743 ESTABLISHED
tcp4 0 0 127.0.0.1.8090 127.0.0.1.61742 ESTABLISHED
tcp4 0 0 127.0.0.1.61744 127.0.0.1.8090 SYN_SENT
tcp4 0 0 127.0.0.1.61748 127.0.0.1.8090 ESTABLISHED
tcp4 0 0 127.0.0.1.61743 127.0.0.1.8090 ESTABLISHED
tcp4 0 0 127.0.0.1.61742 127.0.0.1.8090 ESTABLISHED
tcp4 0 0 127.0.0.1.61749 127.0.0.1.8090 SYN_SENT
tcp4 0 0 127.0.0.1.61747 127.0.0.1.8090 SYN_SENT
tcp4 0 0 127.0.0.1.61750 127.0.0.1.8090 SYN_SENT
tcp4 0 0 127.0.0.1.61751 127.0.0.1.8090 SYN_SENT
tcp4 0 0 127.0.0.1.61745 127.0.0.1.8090 SYN_SENT
tcp4 0 0 127.0.0.1.61746 127.0.0.1.8090 SYN_SENT
tcp46 0 0 *.8090 . LISTEN

四##

tomcat配置：

这次改为发了101个请求，结果连接状态都为ESTABLISHED。

做这几个实验，主要是读文档的时候没有弄清楚acceptCount是当maxThreads达到最大值时起作用还是当maxConnections达到最大值时起作用。

最后通过以上得出结论：##

1）tomcat可接受最大连接数为maxConnections+acceptCount；
2）当maxConnections="0"，maxConnections的值等于maxThreads；
3）当acceptCount="0"，acceptCount的值不是默认的100，具体多少取决操作系统的配置（后面看下tomcat的源码）。
4）maxThreads就是真正同时在处理请求的业务线程（可以通过看stack日志确定）。

其实上面这些通过看tomcat的源码最为准确了，做这个测试主要是为了打发下时间。另外对tcp的连接状态不太清楚的可以看下tcp/ip详解的第18章。