max-http-header-size设置

server.max-http-header-size=999999999 //953m

JVM参数配置

-Xms800m -Xmx800m

写一个rest api

@RestController("action")
public class HttpHandler {
    @PostMapping("/get")
    public String get() {
        return "get";
    }
}

post访问

 服务端OOM

java.lang.OutOfMemoryError: Java heap space
    at java.nio.HeapByteBuffer.<init>(HeapByteBuffer.java:57) ~[na:1.8.0_152]
   at java.nio.ByteBuffer.allocate(ByteBuffer.java:335) ~[na:1.8.0_152]
    at org.apache.coyote.http11.Http11OutputBuffer.<init>(Http11OutputBuffer.java:107) ~[tomcat-embed-core-9.0.33.jar:9.0.33]
    at org.apache.coyote.http11.Http11Processor.<init>(Http11Processor.java:162) ~[tomcat-embed-core-9.0.33.jar:9.0.33]
    at org.apache.coyote.http11.AbstractHttp11Protocol.createProcessor(AbstractHttp11Protocol.java:990) ~[tomcat-embed-core-9.0.33.jar:9.0.33]
    at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:853) ~[tomcat-embed-core-9.0.33.jar:9.0.33]
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1594) [tomcat-embed-core-9.0.33.jar:9.0.33]
    at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49) [tomcat-embed-core-9.0.33.jar:9.0.33]
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) [na:1.8.0_152]
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) [na:1.8.0_152]
    at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) [tomcat-embed-core-9.0.33.jar:9.0.33]
    at java.lang.Thread.run(Thread.java:748) [na:1.8.0_152]

debug，分析

而jvm堆的最大大小是800m，第一时间就oom

其实http11InputBuffer也要初始化一个buffer，这个同样要占很多内存。

 为了演示，本文例子比较极端，堆的大小直接小于buffer的大小，一次请求就oom，一般情况下就算堆的大小大于buffer的大小，如果这个max-size设置的不合理过大，随着请求并发的上升，buffer也会占用大量内存，从而导致oom。

max-size设置过小

 异常Request header is too large，导致post的header存不下来，要根据实际情况进行设置这个值。