JMeter与Spring Boot并发线程2000限制原因排查

本地同时运行JMeter与简易Spring Boot Web服务器时，出现异常：JMeter设置2000个并发线程时所有请求正常，设置为2001个时多数请求失败，报错HttpHostConnectException，请求未到达Spring Boot应用。需要明确2000这个临界值的来源及相关限制配置。

环境配置 #

• Windows 11
• Java 21
• Spring Boot 3.2.0-RC1（3.1.5版本存在同样问题）
• Apache JMeter 5.6.2
• 16核CPU
• 64GB内存

Spring Boot代码 #

import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;

@RestController
@RequestMapping("/person")
public class PersonController {
    @GetMapping
    public String getPerson() {
        return "John";
    }
}

application.properties配置 #

server.port=9001
server.tomcat.threads.max=10000
server.tomcat.max-connections=10000
server.tomcat.accept-count=10000

Spring Boot启动日志 #

MyApplication                            : Starting MyApplication using Java 21 with PID 71328
MyApplication                            : No active profile set, falling back to 1 default profile: "default"
o.s.b.w.embedded.tomcat.TomcatWebServer  : Tomcat initialized with port 9001 (http)
o.apache.catalina.core.StandardService   : Starting service [Tomcat]
o.apache.catalina.core.StandardEngine    : Starting Servlet engine: [Apache Tomcat/10.1.15]
o.a.c.c.C.[Tomcat].[localhost].[/]       : Initializing Spring embedded WebApplicationContext
w.s.c.ServletWebServerApplicationContext : Root WebApplicationContext: initialization completed in 671 ms
o.s.b.w.embedded.tomcat.TomcatWebServer  : Tomcat started on port 9001 (http) with context path ''
MyApplication                            : Started MyApplication in 1.302 seconds (process running for 1.812)
o.a.c.c.C.[Tomcat].[localhost].[/]       : Initializing Spring DispatcherServlet 'dispatcherServlet'
o.s.web.servlet.DispatcherServlet        : Initializing Servlet 'dispatcherServlet'
o.s.web.servlet.DispatcherServlet        : Completed initialization in 2 ms

JMeter相关信息 #

• JMeter请求配置：单HTTP GET请求，目标地址为localhost:9001/person
• 2000并发线程结果：所有请求成功，无失败
• 2001并发线程结果：多数请求失败，报错连接被拒绝

JMeter报错信息 #

org.apache.http.conn.HttpHostConnectException: Connect to localhost:9001 [localhost/127.0.0.1, localhost/0:0:0:0:0:0:0:1] failed: Connection refused: connect
    at org.apache.http.impl.conn.DefaultHttpClientConnectionOperator.connect(DefaultHttpClientConnectionOperator.java:156)
    at org.apache.jmeter.protocol.http.sampler.HTTPHC4Impl$JMeterDefaultHttpClientConnectionOperator.connect(HTTPHC4Impl.java:409)
    at org.apache.http.impl.conn.PoolingHttpClientConnectionManager.connect(PoolingHttpClientConnectionManager.java:376)
    at org.apache.http.impl.execchain.MainClientExec.establishRoute(MainClientExec.java:393)
    at org.apache.http.impl.execchain.MainClientExec.execute(MainClientExec.java:236)
    at org.apache.http.impl.execchain.ProtocolExec.execute(ProtocolExec.java:186)
    at org.apache.http.impl.execchain.RetryExec.execute(RetryExec.java:89)
    at org.apache.http.impl.execchain.RedirectExec.execute(RedirectExec.java:110)
    at org.apache.http.impl.client.InternalHttpClient.doExecute(InternalHttpClient.java:185)
    at org.apache.http.impl.client.CloseableHttpClient.execute(CloseableHttpClient.java:83)
    at org.apache.jmeter.protocol.http.sampler.HTTPHC4Impl.executeRequest(HTTPHC4Impl.java:940)
    at org.apache.jmeter.protocol.http.sampler.HTTPHC4Impl.sample(HTTPHC4Impl.java:651)
    at org.apache.jmeter.protocol.http.sampler.HTTPSamplerProxy.sample(HTTPSamplerProxy.java:66)
    at org.apache.jmeter.protocol.http.sampler.HTTPSamplerBase.sample(HTTPSamplerBase.java:1311)
    at org.apache.jmeter.protocol.http.sampler.HTTPSamplerBase.sample(HTTPSamplerBase.java:1300)
    at org.apache.jmeter.threads.JMeterThread.doSampling(JMeterThread.java:651)
    at org.apache.jmeter.threads.JMeterThread.executeSamplePackage(JMeterThread.java:570)
    at org.apache.jmeter.threads.JMeterThread.processSampler(JMeterThread.java:501)
    at org.apache.jmeter.threads.JMeterThread.run(JMeterThread.java:268)
    at java.base/java.lang.Thread.run(Thread.java:1583)
Caused by: java.net.ConnectException: Connection refused: connect
    at java.base/sun.nio.ch.Net.connect0(Native Method)
    at java.base/sun.nio.ch.Net.connect(Net.java:589)
    at java.base/sun.nio.ch.Net.connect(Net.java:578)
    at java.base/sun.nio.ch.NioSocketImpl.connect(NioSocketImpl.java:583)
    at java.base/java.net.SocksSocketImpl.connect(SocksSocketImpl.java:327)
    at java.base/java.net.Socket.connect(Socket.java:751)
    at org.apache.http.conn.socket.PlainConnectionSocketFactory.connectSocket(PlainConnectionSocketFactory.java:75)
    at org.apache.http.impl.conn.DefaultHttpClientConnectionOperator.connect(DefaultHttpClientConnectionOperator.java:142)
    ... 19 more

原因分析与解决方案 #

1. 核心原因：Windows系统层面的TCP限制 #

2000的临界值并非来自Spring Boot/Tomcat配置（你已将Tomcat的连接、线程、等待队列设为10000），而是Windows系统默认的TCP连接相关限制，具体有两个可能：

（1）本地临时端口耗尽

JMeter与Spring Boot在同一台机器上，每个JMeter线程会占用一个本地临时端口去连接服务端的9001端口。Windows默认临时端口范围是1024-5000，共3976个可用端口，但TCP连接关闭后会进入TIME_WAIT状态（默认保留240秒），短连接场景下大量端口被占用无法复用，当并发达到2000时，可用端口接近耗尽，新连接无法创建，导致Connection refused。

验证方式：运行netstat -ano | findstr :9001，查看是否有大量TIME_WAIT状态的连接。

解决方法：扩大临时端口范围，以管理员身份运行命令：

netsh int ipv4 set dynamicport tcp start=1024 num=64511
netsh int ipv6 set dynamicport tcp start=1024 num=64511

执行后重启机器生效。

（2）TCP全连接队列限制

Windows系统默认的MaxConnectionsBacklog（TCP全连接队列大小）为2000，即使Tomcat设置了accept-count=10000，系统层面的限制会优先生效。当并发请求超过2000时，系统直接拒绝新连接，服务端无法接收请求，导致报错。

验证方式：查看注册表项HKLM\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\MaxConnectionsBacklog（若不存在则为默认值2000）。

解决方法：

1. 打开注册表编辑器，定位到上述路径；
2. 新建或修改DWORD类型的MaxConnectionsBacklog，设置值为10000（与Tomcat的accept-count保持一致）；
3. 重启机器生效。

2. 辅助优化：JMeter连接复用 #

开启JMeter的连接复用，减少端口占用：

• 在HTTP请求的「高级」选项中勾选「Use keepalive」；
• 调整JMeter的HTTP连接池配置，在jmeter.properties中修改：

  httpclient4.max_total=20000
  httpclient4.max_per_route=20000
  

3. 确认Tomcat配置生效 #

通过JMX工具（如JConsole）连接Spring Boot应用，查看Tomcat的maxConnections、maxThreads等指标，确认配置已正确加载。

内容的提问来源于stack exchange，提问作者Honza Zidek