IPv6调用java后端接口报错：java.net.SocketException: Protocol family unavailable

　　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::8A:AF:9E:
          inet addr:192.168.11.11  Bcast:192.168.11.111  Mask:255.255.255.0
          inet6 addr: fe80::2e76:8aff:feaf:9e82/ Scope:Link
          inet6 addr: ::8f1:::/ Scope:Global
          UP BROADCAST RUNNING MULTICAST  MTU:  Metric:
          RX packets: errors: dropped: overruns: frame:
          TX packets: errors: dropped: overruns: carrier:
          collisions: txqueuelen:
          RX bytes: (244895.4 Mb)  TX bytes: (317177046.3 Mb)
          Memory:f9ec0000-f9ee0000 

eth2      Link encap:Ethernet  HWaddr 9C:8E::2A:EE:
          inet addr:11.111.11.11  Bcast:11.111.11.11  Mask:255.255.255.192
          inet6 addr: fe80::9e8e:99ff:fe2a:ee94/ Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:  Metric:
          RX packets: errors: dropped: overruns: frame:
          TX packets: errors: dropped: overruns: carrier:
          collisions: txqueuelen:
          RX bytes: (817469.0 Mb)  TX bytes: (904463.4 Mb)
          Interrupt: Memory:fc000000-fc012800 

lo        Link encap:Local Loopback
          inet addr:127.0.0.1  Mask:255.0.0.0
          inet6 addr: ::/ Scope:Host
          UP LOOPBACK RUNNING  MTU:  Metric:
          RX packets: errors: dropped: overruns: frame:
          TX packets: errors: dropped: overruns: carrier:
          collisions: txqueuelen:
          RX bytes: (3356226.2 Mb)  TX bytes: (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]: >>>
<<<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:)
        at java.net.AbstractPlainSocketImpl.connectToAddress(AbstractPlainSocketImpl.java:)
        at java.net.AbstractPlainSocketImpl.connect(AbstractPlainSocketImpl.java:)
        at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:)
        at java.net.Socket.connect(Socket.java:)
        at java.net.Socket.connect(Socket.java:)
        at java.net.Socket.<init>(Socket.java:)
        at java.net.Socket.<init>(Socket.java:)
        at org.apache.commons.httpclient.protocol.DefaultProtocolSocketFactory.createSocket(DefaultProtocolSocketFactory.java:)
        at org.apache.commons.httpclient.protocol.DefaultProtocolSocketFactory.createSocket(DefaultProtocolSocketFactory.java:)
        at org.apache.commons.httpclient.HttpConnection.open(HttpConnection.java:)
        at org.apache.commons.httpclient.MultiThreadedHttpConnectionManager$HttpConnectionAdapter.open(MultiThreadedHttpConnectionManager.java:)
        at org.apache.commons.httpclient.HttpMethodDirector.executeWithRetry(HttpMethodDirector.java:)
        at org.apache.commons.httpclient.HttpMethodDirector.executeMethod(HttpMethodDirector.java:)
        at org.apache.commons.httpclient.HttpClient.executeMethod(HttpClient.java:)
        at org.apache.commons.httpclient.HttpClient.executeMethod(HttpClient.java:)
        at com.migu.reading.common.utils.HttpTools.sendHttpRequest(HttpTools.java:)
        at com.migu.reading.servlet.assistant.InterfaceServlet.service(InterfaceServlet.java:)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:)
        at com.migu.reading.auditweb.filter.AuditFilter.doFilter(AuditFilter.java:)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:)
        at com.alibaba.druid.support.http.WebStatFilter.doFilter(WebStatFilter.java:)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:)
        at com.migu.reading.filter.CharacterEncodingFilter.doFilter(CharacterEncodingFilter.java:)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:)
        at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:)
        at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:)
        at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:)
        at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:)
        at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:)
        at com.huawei.openas.monitor.valve.RequestCounterValve.invoke(RequestCounterValve.java:)
        at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:)
        at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:)
        at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:)
        at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:)
        at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:)
        at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:)
        at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:)
        at java.lang.Thread.run(Thread.java:)
<<<Retrying request >>>
<<<Open connection to [fe80::9e8e:99ff:fe2a:ee94]: >>>
<<<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");