感谢 shuai.xie 提出的这段漏掉的内容，这里补充一下。
这里b的类型声明并不是必须的。因为我们已经告诉Scala返回类型为B => C，Scala会从上下文获知b的类型，方法的实现部分只需要写为 b => f(a,b)就可以了。如果Scala能够推断出函数字面量的类型，就可以省略掉它的类型声明。
相关阅读：《Scala函数式编程》中文版勘误
《Scala函数式编程》中文版勘误2
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Monthly Archives: February 2017
记录几个实践中的问题
1) nginx禁止对写操作timeout时retry
以前遇到的一个case，业务那边说一笔请求从nginx端发送给后端tomcat了2次（落在两个不同的tomcat节点上）。后来发现是nginx发给后端节点timeout，然后做了重试，发给了另一个节点。默认情况下nginx对后端error 和 timeout 都会做retry，可以明确的禁止在timeout的情况下禁止retry。当然如果集群读写分离的话，对于只读集群retry是无所谓的，但对于写确实存在问题。
2) kafka重启时因为数据日志文件名被人重命名过而导致启动失败
启动kafka broker的时候，会重新load之前的每个topic的数据，正常情况下会提示每个topic恢复完成。
INFO Recovering unflushed segment 588022 in log xxx-topic-0. (kafka.log.Log)
INFO Completed load of log xxx-topic-0 with log end offset 590676 (kafka.log.Log)但当有些topic下的数据恢复失败的时候，会导致broker关闭，异常如下
ERROR There was an error in one of the threads during logs loading: java.lang.NumberFormatException: For input string: "test" (kafka.log.LogManager)
FATAL [Kafka Server 3], Fatal error during KafkaServer startup. Prepare to shutdown (kafka.server.KafkaServer)
java.lang.NumberFormatException: For input string: "test"
at java.lang.NumberFormatException.forInputString(NumberFormatException.java:65)
at java.lang.Long.parseLong(Long.java:589)
at java.lang.Long.parseLong(Long.java:631)
at scala.collection.immutable.StringLike$class.toLong(StringLike.scala:251)
at scala.collection.immutable.StringOps.toLong(StringOps.scala:30)
at kafka.log.Log$$anonfun$loadSegments$4.apply(Log.scala:152)
at kafka.log.Log$$anonfun$loadSegments$4.apply(Log.scala:141)
at scala.collection.TraversableLike$WithFilter$$anonfun$foreach$1.apply(TraversableLike.scala:778)
at scala.collection.IndexedSeqOptimized$class.foreach(IndexedSeqOptimized.scala:33)
at scala.collection.mutable.ArrayOps$ofRef.foreach(ArrayOps.scala:186)
at scala.collection.TraversableLike$WithFilter.foreach(TraversableLike.scala:777)
at kafka.log.Log.loadSegments(Log.scala:141)
at kafka.log.Log.<init>(Log.scala:67)
at kafka.log.LogManager$$anonfun$loadLogs$2$$anonfun$3$$anonfun$apply$7$$anonfun$apply$1.apply$mcV$sp(LogManager.scala:142)
at kafka.utils.Utils$$anon$1.run(Utils.scala:54)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
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)这是因为某个目录下，存在一个 test.log 的文件
$ ls mytopic-0/
00000000000000000485.index 00000000000000000485.log 00000000000000000568.index 00000000000000000568.log test.log看上去这个 test.log 当时是把 00…log 给拷贝了一个，然后用编辑器去查看内容。而事后忘了清理掉，导致重启时把这个文件当成一个畸形文件了。因为kafka broker要求所有数据文件名称都是Long类型的。
3) 又一个actor阻塞的例子
在我自己的mac上测试的时候，一切正常，部署到dev环境就严重超时。jstack观察发现又是误用阻塞操作导致所有actor的线程都被阻塞所致，当时 EventProcessor 这个 Router 背后的实例数设置的是40，而这台dev环境的linux只有2核，根据当时akka的配置里的并发因子算出并发线程数是32，所以32个线程基本都被 eventProcessor 的40个actor全给占用了，因为它是不断发消息轮询的（我的mac是8核，运行时的线程数要大于40不会发生全部被阻塞的情况）。解决方式，一方面调大并发因子，把线城数提升上去，另一方面控制 eventProcessor 的实例数，不让它的阻塞操作影响到其他actor。(其实根上是没设计好，没有隔离阻塞操作，只不过这正好是个小应用，不需要过多考虑。)
有道词典的异常堆栈
碰巧抓住了有道词典的异常页面，从堆栈里能看出他们的一些技术栈，他们使用的是Resin-3.0.21，一个十多年前曾流行过现在不太主流的应用容器，但在网易、搜狐、人人等公司这个容器依然很常见。他们用http去访问cassandra?
java.lang.NoSuchFieldError: INSTANCE
at org.apache.http.conn.ssl.SSLConnectionSocketFactory.<clinit>(SSLConnectionSocketFactory.java:144)
at org.apache.http.client.fluent.Executor.<clinit>(Executor.java:78)
at org.apache.http.client.fluent.Request.execute(Request.java:177)
at toolbox.cassandra.client.CassandraClient.httpGet(CassandraClient.java:47)
at toolbox.cassandra.client.CassandraClient.init(CassandraClient.java:129)
at toolbox.cassandra.client.CassandraClient.getInstance(CassandraClient.java:75)
at outfox.dict.front.data.ugc.AsynUgcClient.<clinit>(AsynUgcClient.java:88)
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:39)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:27)
at java.lang.reflect.Constructor.newInstance(Constructor.java:513)
at org.springframework.beans.BeanUtils.instantiateClass(BeanUtils.java:148)
at org.springframework.beans.factory.support.SimpleInstantiationStrategy.instantiate(SimpleInstantiationStrategy.java:87)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.instantiateBean(AbstractAutowireCapableBeanFactory.java:1000)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:953)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:487)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:458)
at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:295)
at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:223)
at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:292)
at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:194)
at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:628)
at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:932)
at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:479)
at org.springframework.web.servlet.FrameworkServlet.configureAndRefreshWebApplicationContext(FrameworkServlet.java:651)
at org.springframework.web.servlet.FrameworkServlet.createWebApplicationContext(FrameworkServlet.java:599)
at org.springframework.web.servlet.FrameworkServlet.createWebApplicationContext(FrameworkServlet.java:665)
at org.springframework.web.servlet.FrameworkServlet.initWebApplicationContext(FrameworkServlet.java:518)
at org.springframework.web.servlet.FrameworkServlet.initServletBean(FrameworkServlet.java:459)
at org.springframework.web.servlet.HttpServletBean.init(HttpServletBean.java:136)
at javax.servlet.GenericServlet.init(GenericServlet.java:69)
at outfox.dict.front.web.DictServlet.init(DictServlet.java:57)
at com.caucho.server.dispatch.ServletConfigImpl.createServletImpl(ServletConfigImpl.java:646)
at com.caucho.server.dispatch.ServletConfigImpl.createServlet(ServletConfigImpl.java:587)
at com.caucho.server.dispatch.ServletManager.init(ServletManager.java:154)
at com.caucho.server.webapp.Application.start(Application.java:1654)
at com.caucho.server.deploy.DeployController.startImpl(DeployController.java:621)
at com.caucho.server.deploy.StartAutoRedeployAutoStrategy.startOnInit(StartAutoRedeployAutoStrategy.java:72)
at com.caucho.server.deploy.DeployController.startOnInit(DeployController.java:509)
at com.caucho.server.deploy.DeployContainer.start(DeployContainer.java:153)
at com.caucho.server.webapp.ApplicationContainer.start(ApplicationContainer.java:670)
at com.caucho.server.host.Host.start(Host.java:420)
at com.caucho.server.deploy.DeployController.startImpl(DeployController.java:621)
at com.caucho.server.deploy.StartAutoRedeployAutoStrategy.startOnInit(StartAutoRedeployAutoStrategy.java:72)
at com.caucho.server.deploy.DeployController.startOnInit(DeployController.java:509)
at com.caucho.server.deploy.DeployContainer.start(DeployContainer.java:153)
at com.caucho.server.host.HostContainer.start(HostContainer.java:504)
at com.caucho.server.resin.ServletServer.start(ServletServer.java:971)
at com.caucho.server.deploy.DeployController.startImpl(DeployController.java:621)
at com.caucho.server.deploy.AbstractDeployControllerStrategy.start(AbstractDeployControllerStrategy.java:56)
at com.caucho.server.deploy.DeployController.start(DeployController.java:517)
at com.caucho.server.resin.ResinServer.start(ResinServer.java:546)
at com.caucho.server.resin.Resin.init(Resin.java)
at com.caucho.server.resin.Resin.main(Resin.java:625)OneApm的问题
1) 我们没有使用它的大数据版本，普通版本在压测情况下服务器端如果吞吐不够，agent端会缓存大量的数据导致full gc，这里它占据了1G的内存
2) 对HttpURLConnection拦截时可能发生空指针异常，导致业务线程终止
java.lang.NullPointerException
at com.blueware.monitor.bridge.reflect.ClassReflection$2.run(ClassReflection.java:30) ~[?:?]
at com.blueware.monitor.bridge.reflect.ClassReflection$2.run(ClassReflection.java:28) ~[?:?]
at java.security.AccessController.doPrivileged(Native Method) ~[?:1.8.0_65]
at com.blueware.monitor.bridge.reflect.ClassReflection.loadClass(ClassReflection.java:28) ~[?:?]
at sun.net.www.protocol.http.HttpURLConnection.getInputStream(HttpURLConnection.java:1441) ~[?:1.8.0_65]
at sun.net.www.protocol.http.HttpURLConnection.getHeaderField(HttpURLConnection.java:2979) ~[?:1.8.0_65]
at java.net.HttpURLConnection.getResponseCode(HttpURLConnection.java:489) ~[?:1.8.0_65]
at com...