目前需求是java后端的接口需要支持IPv6。先确认linux机器已经绑定了IPv6：
CMREAD-SV43 apache-tomcat/bin> ifconfig eth0 Link encap:Ethernet HWaddr 2C:76:8A:AF:9E:82 inet addr:192.168.11.11 Bcast:192.168.11.111 Mask:255.255.255.0 inet6 addr: fe80::2e76:8aff:feaf:9e82/64 Scope:Link inet6 addr: 2409:8028:8f1:1202::69/64 Scope:Global UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:3032806443 errors:0 dropped:332 overruns:0 frame:0 TX packets:320990686207 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:1000 RX bytes:256791451048 (244895.4 Mb) TX bytes:332584238528831 (317177046.3 Mb) Memory:f9ec0000-f9ee0000 eth2 Link encap:Ethernet HWaddr 9C:8E:99:2A:EE:94 inet addr:11.111.11.11 Bcast:11.111.11.11 Mask:255.255.255.192 inet6 addr: fe80::9e8e:99ff:fe2a:ee94/64 Scope:Link UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:2905011525 errors:0 dropped:0 overruns:0 frame:0 TX packets:3243480696 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:1000 RX bytes:857178438283 (817469.0 Mb) TX bytes:948398682892 (904463.4 Mb) Interrupt:36 Memory:fc000000-fc012800 lo Link encap:Local Loopback inet addr:127.0.0.1 Mask:255.0.0.0 inet6 addr: ::1/128 Scope:Host UP LOOPBACK RUNNING MTU:16436 Metric:1 RX packets:62415690585 errors:0 dropped:0 overruns:0 frame:0 TX packets:62415690585 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:0 RX bytes:3519258276730 (3356226.2 Mb) TX bytes:3519258276730 (3356226.2 Mb)
我们拿网卡二（eth2）的IPv6去调接口，这里注意需要给IPv6地址加上左右中括号，即以这种格式去调
http://[fe80::9e8e:99ff:fe2a:ee94]:9088/接口名
结果java报错了：
<<<HttpConnectionManager.getConnection: config = HostConfiguration[host=http://[fe80::9e8e:99ff:fe2a:ee94]:9088], timeout = 500 >>> <<<Allocating new connection, hostConfig=HostConfiguration[host=http://[fe80::9e8e:99ff:fe2a:ee94]:9088] >>> <<<Open connection to [fe80::9e8e:99ff:fe2a:ee94]:9088 >>> <<<Closing the connection. >>> <<<I/O exception (java.net.SocketException) caught when processing request: Protocol family unavailable >>> <<<Protocol family unavailable >>> java.net.SocketException: Protocol family unavailable at java.net.PlainSocketImpl.socketConnect(Native Method) at java.net.AbstractPlainSocketImpl.doConnect(AbstractPlainSocketImpl.java:350) at java.net.AbstractPlainSocketImpl.connectToAddress(AbstractPlainSocketImpl.java:206) at java.net.AbstractPlainSocketImpl.connect(AbstractPlainSocketImpl.java:188) at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:392) at java.net.Socket.connect(Socket.java:589) at java.net.Socket.connect(Socket.java:538) at java.net.Socket.<init>(Socket.java:434) at java.net.Socket.<init>(Socket.java:286) at org.apache.commons.httpclient.protocol.DefaultProtocolSocketFactory.createSocket(DefaultProtocolSocketFactory.java:80) at org.apache.commons.httpclient.protocol.DefaultProtocolSocketFactory.createSocket(DefaultProtocolSocketFactory.java:122) at org.apache.commons.httpclient.HttpConnection.open(HttpConnection.java:707) at org.apache.commons.httpclient.MultiThreadedHttpConnectionManager$HttpConnectionAdapter.open(MultiThreadedHttpConnectionManager.java:1361) at org.apache.commons.httpclient.HttpMethodDirector.executeWithRetry(HttpMethodDirector.java:387) at org.apache.commons.httpclient.HttpMethodDirector.executeMethod(HttpMethodDirector.java:171) at org.apache.commons.httpclient.HttpClient.executeMethod(HttpClient.java:397) at org.apache.commons.httpclient.HttpClient.executeMethod(HttpClient.java:323) at com.migu.reading.common.utils.HttpTools.sendHttpRequest(HttpTools.java:185) at com.migu.reading.servlet.assistant.InterfaceServlet.service(InterfaceServlet.java:163) at javax.servlet.http.HttpServlet.service(HttpServlet.java:728) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:305) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210) at com.migu.reading.auditweb.filter.AuditFilter.doFilter(AuditFilter.java:169) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:243) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210) at com.alibaba.druid.support.http.WebStatFilter.doFilter(WebStatFilter.java:123) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:243) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210) at com.migu.reading.filter.CharacterEncodingFilter.doFilter(CharacterEncodingFilter.java:27) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:243) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210) at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:222) at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:123) at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:472) at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:171) at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:99) at com.huawei.openas.monitor.valve.RequestCounterValve.invoke(RequestCounterValve.java:128) at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:936) at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:936) at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:118) at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:407) at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1004) at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:589) at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:310) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617) at java.lang.Thread.run(Thread.java:745) <<<Retrying request >>> <<<Open connection to [fe80::9e8e:99ff:fe2a:ee94]:9088 >>> <<<Closing the connection. >>> <<<I/O exception (java.net.SocketException) caught when processing request: Protocol family unavailable >>>
按理java是自动支持IPv6的，为啥还报错呢？看来应该是环境主动配置了针对IPv4的开关，一找发现在tomcat的bin目录下果然有一个setvmargs.sh的文件设置了java.net.preferIPv4Stack系统属性：
JAVA_OPTS="$JAVA_OPTS -server -Xmn2457M -Xms6384M -Xmx6384M -Djava.net.preferIPv4Stack=true -Djava.awt.headless=true -XX:PermSize=5120M -XX:MaxPermSize=5120M -XX:+UseConcMarkSweepGC -XX:+UseParNewGC -XX:+CMSParallelRemarkEnabled -XX:+UseCMSCompactAtFullCollection -XX:CMSFullGCsBeforeCompaction=0 -XX:+CMSClassUnloadingEnabled -XX:+UseFastAccessorMethods -XX:+UseCMSInitiatingOccupancyOnly -XX:SurvivorRatio=4 -XX:MaxTenuringThreshold=5 -XX:CMSInitiatingOccupancyFraction=70 -XX:SoftRefLRUPolicyMSPerMB=0 -Xloggc:gc.log -XX:+PrintGCDetails -Dcom.sun.management.jmxremote=true -Dcom.sun.management.jmxremote.authenticate=false -Dcom.sun.management.jmxremote.port=29331 -Dcom.sun.management.jmxremote.ssl=false"
该参数默认是false，在支持IPv6的双栈系统上，使用Java的Socket会默认通过底层native方法创建一个IPv6 Socket，这个IPv6 Socket可以同时支持和IPv4或IPv6主机通信。当TCP客户端java.net.preferIPv4Stack设置为true时，如果想创建一个host为IPv6的Socket，会抛出异常java.net.SocketException: Protocol family unavailable，设置为false时则程序可以正常运行。
类似的，还有另一个系统属性java.net.preferIPv6Addresses，它默认也是false，但值却跟上面属性相反，它支持的是IPv6，配置true时才支持。我们将java.net.preferIPv4Stack改为false后重启系统，问题解决。
对了，以上两个系统属性也可以在代码里指定：
System.setProperty("java.net.preferIPv4Stack", "true");