Here's yours modified to wait for close before testing the list of beans.
The output of that I get on my Linux machine ...
java.awt.graphicsenv = sun.awt.X11GraphicsEnvironment
java.awt.printerjob = sun.print.PSPrinterJob
java.class.path = /home/joakim/code/jetty/Playground/target/classes:/home/joakim/.m2/repository/org/jboss/weld/servlet/weld-servlet/2.2.9.Final/weld-servlet-2.2.9.Final.jar:/home/joakim/.m2/repository/javax/websocket/javax.websocket-api/1.0/javax.websocket-api-1.0.jar:/home/joakim/.m2/repository/javax/servlet/javax.servlet-api/3.1.0/javax.servlet-api-3.1.0.jar:/home/joakim/.m2/repository/org/eclipse/jetty/jetty-deploy/9.3.7.RC0/jetty-deploy-9.3.7.RC0.jar:/home/joakim/.m2/repository/org/eclipse/jetty/jetty-xml/9.3.7.RC0/jetty-xml-9.3.7.RC0.jar:/home/joakim/.m2/repository/org/eclipse/jetty/jetty-http/9.3.7.RC0/jetty-http-9.3.7.RC0-tests.jar:/home/joakim/.m2/repository/org/eclipse/jetty/jetty-util/9.3.7.RC0/jetty-util-9.3.7.RC0.jar:/home/joakim/.m2/repository/org/eclipse/jetty/jetty-webapp/9.3.7.RC0/jetty-webapp-9.3.7.RC0.jar:/home/joakim/.m2/repository/org/eclipse/jetty/jetty-servlet/9.3.7.RC0/jetty-servlet-9.3.7.RC0.jar:/home/joakim/.m2/repository/org/eclipse/jetty/jetty-servlets/9.3.7.RC0/jetty-servlets-9.3.7.RC0.jar:/home/joakim/.m2/repository/org/eclipse/jetty/jetty-continuation/9.3.7.RC0/jetty-continuation-9.3.7.RC0.jar:/home/joakim/.m2/repository/org/eclipse/jetty/jetty-http/9.3.7.RC0/jetty-http-9.3.7.RC0.jar:/home/joakim/.m2/repository/org/eclipse/jetty/jetty-io/9.3.7.RC0/jetty-io-9.3.7.RC0.jar:/home/joakim/.m2/repository/org/eclipse/jetty/jetty-annotations/9.3.7.RC0/jetty-annotations-9.3.7.RC0.jar:/home/joakim/.m2/repository/org/eclipse/jetty/jetty-plus/9.3.7.RC0/jetty-plus-9.3.7.RC0.jar:/home/joakim/.m2/repository/org/eclipse/jetty/jetty-jndi/9.3.7.RC0/jetty-jndi-9.3.7.RC0.jar:/home/joakim/.m2/repository/javax/annotation/javax.annotation-api/1.2/javax.annotation-api-1.2.jar:/home/joakim/.m2/repository/org/ow2/asm/asm/5.0.1/asm-5.0.1.jar:/home/joakim/.m2/repository/org/ow2/asm/asm-commons/5.0.1/asm-commons-5.0.1.jar:/home/joakim/.m2/repository/org/ow2/asm/asm-tree/5.0.1/asm-tree-5.0.1.jar:/home/joakim/.m2/repository/org/eclipse/jetty/websocket/javax-websocket-server-impl/9.3.7.RC0/javax-websocket-server-impl-9.3.7.RC0.jar:/home/joakim/.m2/repository/org/eclipse/jetty/websocket/javax-websocket-client-impl/9.3.7.RC0/javax-websocket-client-impl-9.3.7.RC0.jar:/home/joakim/.m2/repository/org/eclipse/jetty/websocket/websocket-client/9.3.7.RC0/websocket-client-9.3.7.RC0.jar:/home/joakim/.m2/repository/org/eclipse/jetty/websocket/websocket-server/9.3.7.RC0/websocket-server-9.3.7.RC0.jar:/home/joakim/.m2/repository/org/eclipse/jetty/websocket/websocket-common/9.3.7.RC0/websocket-common-9.3.7.RC0.jar:/home/joakim/.m2/repository/org/eclipse/jetty/websocket/websocket-api/9.3.7.RC0/websocket-api-9.3.7.RC0.jar:/home/joakim/.m2/repository/org/eclipse/jetty/websocket/websocket-servlet/9.3.7.RC0/websocket-servlet-9.3.7.RC0.jar:/home/joakim/.m2/repository/org/eclipse/jetty/jetty-rewrite/9.3.7.RC0/jetty-rewrite-9.3.7.RC0.jar:/home/joakim/.m2/repository/org/eclipse/jetty/jetty-server/9.3.7.RC0/jetty-server-9.3.7.RC0.jar:/home/joakim/.m2/repository/org/eclipse/jetty/jetty-client/9.3.7.RC0/jetty-client-9.3.7.RC0.jar:/home/joakim/.m2/repository/org/eclipse/jetty/jetty-proxy/9.3.7.RC0/jetty-proxy-9.3.7.RC0.jar:/home/joakim/.m2/repository/org/eclipse/jetty/jetty-servlet/9.3.7.RC0/jetty-servlet-9.3.7.RC0-tests.jar:/home/joakim/.m2/repository/org/eclipse/jetty/jetty-security/9.3.7.RC0/jetty-security-9.3.7.RC0.jar:/home/joakim/.m2/repository/org/eclipse/jetty/toolchain/jetty-test-helper/2.9/jetty-test-helper-2.9.jar:/home/joakim/.m2/repository/junit/junit/4.11/junit-4.11.jar:/home/joakim/.m2/repository/org/hamcrest/hamcrest-core/1.3/hamcrest-core-1.3.jar:/home/joakim/.m2/repository/org/hamcrest/hamcrest-library/1.3/hamcrest-library-1.3.jar
java.class.version = 52.0
java.endorsed.dirs = /home/joakim/java/jvm/jdk-8u60/jre/lib/endorsed
java.ext.dirs = /home/joakim/java/jvm/jdk-8u60/jre/lib/ext:/usr/java/packages/lib/ext
java.home = /home/joakim/java/jvm/jdk-8u60/jre
java.io.tmpdir = /tmp
java.library.path = /usr/lib/jni:/usr/java/packages/lib/amd64:/usr/lib64:/lib64:/lib:/usr/lib
java.runtime.version = 1.8.0_60-b27
java.specification.vendor = Oracle Corporation
java.specification.version = 1.8
java.vendor = Oracle Corporation
java.vendor.url = "" href="http://java.oracle.com/">http://java.oracle.com/
java.version = 1.8.0_60
java.vm.specification.vendor = Oracle Corporation
java.vm.specification.version = 1.8
java.vm.vendor = Oracle Corporation
java.vm.version = 25.60-b23
sun.arch.data.model = 64
sun.boot.class.path = /home/joakim/java/jvm/jdk-8u60/jre/lib/resources.jar:/home/joakim/java/jvm/jdk-8u60/jre/lib/rt.jar:/home/joakim/java/jvm/jdk-8u60/jre/lib/sunrsasign.jar:/home/joakim/java/jvm/jdk-8u60/jre/lib/jsse.jar:/home/joakim/java/jvm/jdk-8u60/jre/lib/jce.jar:/home/joakim/java/jvm/jdk-8u60/jre/lib/charsets.jar:/home/joakim/java/jvm/jdk-8u60/jre/lib/jfr.jar:/home/joakim/java/jvm/jdk-8u60/jre/classes
sun.boot.library.path = /home/joakim/java/jvm/jdk-8u60/jre/lib/amd64
sun.cpu.endian = little
sun.cpu.isalist =
sun.desktop = gnome
sun.io.unicode.encoding = UnicodeLittle
sun.java.command = jetty.websocket.JettyWebSocketLeak
sun.java.launcher = SUN_STANDARD
sun.jnu.encoding = UTF-8
sun.management.compiler = HotSpot 64-Bit Tiered Compilers
sun.os.patch.level = unknown
user.country = US
user.dir = /home/joakim/code/jetty/Playground
user.home = /home/joakim
user.language = en
user.timezone =
2016-01-12 09:07:21.879:INFO::main: Logging initialized @182ms
2016-01-12 09:07:21.979:INFO:oejs.Server:main: jetty-9.3.7.RC0
2016-01-12 09:07:22.228:INFO:oejsh.ContextHandler:main: Started o.e.j.s.ServletContextHandler@62bd765{/,null,AVAILABLE}
2016-01-12 09:07:22.236:INFO:oejs.ServerConnector:main: Started ServerConnector@351d0846{HTTP/1.1,[http/1.1]}{
0.0.0.0:9090}
2016-01-12 09:07:22.237:INFO:oejs.Server:main: Started @541ms
printing the leaked server side sessions ...
2016-01-12 09:07:22.353:INFO:oejs.ServerConnector:main: Stopped ServerConnector@351d0846{HTTP/1.1,[http/1.1]}{
0.0.0.0:9090}
2016-01-12 09:07:22.354:INFO:oejsh.ContextHandler:main: Stopped o.e.j.s.ServletContextHandler@62bd765{/,null,UNAVAILABLE}
If I get rid of the 2 lines ...
Then I occasionally (its not 100% of the time) see some (still open) WebSocketSession's lingering as beans on the WebSocketServerFactory.