Kent Tong's personal thoughts on information technology

Problem with equals()

The problem with equals() is that it is difficult to get it right. For example, for a simple class Foo, you may write the equals() method as:

class Foo(val i: Int) {
  override def equals(that: Any) = {
    that match {
      case f: Foo => f.i == i
      case _ => false
    }
  }
}

The problem is that what happens if the other object (“that”) belongs to a subclass of Foo, which may have its own fields? As the equals() method is only comparing the “i” field, it will ignore the other fields and return true prematurely. Below is such a subclass Bar:

class Bar(i: Int, val j: Int) extends Foo(i) {
  override def equals(that: Any) = {
    that match {
      case b: Bar => super.equals(b) && b.j == j
      case _ => false
    }
  }
}

With the erroneous equals() method in Foo, we could get incorrect results:

scala> val f1 = new Foo(2)
f1: Foo = Foo@14c0275

scala> val b1 = new Bar(2, 3)
b1: Bar = Bar@171bc3f

scala> f1.equals(b1)
res0: Boolean = true

scala> b1.equals(f1)
res1: Boolean = false

A solution to the problem

The problem is that the equals() method in Foo is treating the Bar object exactly as a base Foo object, but the equality contract in Bar has changed from that in Foo. Of course, not every subclass of Foo will use a different equality contract; some do and some don’t (by default, we should assume that they don’t). Therefore, the equals() method in Foo should make sure that the “that” object uses the same equality contract as “this”:

object FooEqualityContract {
}

class Foo(val i: Int) {
  //by default all Foo objects and subclass objects use this equality contract
  val equalityContract: Any = FooEqualityContract

  override def equals(that: Any) = {
    that match {
      //make sure the two objects are using the same equality contract
      case f: Foo => f.equalityContract == this.equalityContract && f.i == i
      case _ => false
    }
  }
}

Now, as Bar is using its own equality contract, it should say so:

class Bar(i: Int, val j: Int) extends Foo(i) {
  //tell others that we're using our own equality contract
  override val equalityContract: Any = BarEqualityContract

  override def equals(that: Any) = {
    that match {
      case b: Bar => super.equals(b) && b.j == j
      case _ => false
    }
  }
}

Now, the equals() method in Foo will rightly determine that a Bar object is using a different equality contract and thus will never be equal to a bare Foo object:

scala> val f1 = new Foo(2)
f1: Foo = Foo@34b350

scala> val b1 = new Bar(2, 3)
b1: Bar = Bar@7c28c

scala> f1.equals(b1)
res2: Boolean = false

scala> b1.equals(f1)
res3: Boolean = false

scala> val b2 = new Bar(2, 3)
b2: Bar = Bar@5dd915

scala> b1.equals(b2)
res6: Boolean = true

Of course, it should also work for subclasses that use the same equality contract:

scala> val f2 = new Foo(2) { }
f2: Foo = $anon$1@a594e1

scala> f1.equals(f2)
res9: Boolean = true

scala> f2.equals(f1)
res10: Boolean = true

Scala is truly magical. However, sometimes it is not easy to understand how it performs the magic. Below are some common questions and their answers.

Why some of expressions below work (can be evaluated and printed) but the other don’t?

Math.min  //doesn't work
Math.min _  //works
val f: (Int, Int)=>Int = Math.min  //works

The first expression doesn’t work because Math.min is a method, but a method is not a value in Scala. The second expression works because the underscore asks Scala to convert the method to a function, which is indeed a value in Scala. The third expression also works because when Scala is expecting a function value from the expression but finds a method, it will convert it to a function automatically.

Why some of expressions below work but the other don’t?

List(2, 3, 5) foreach println  //works
List(2, 3, 5) foreach println(_) //doesn't work
List(2, 3, 5) foreach (println(_)) //works

The first one works because foreach is expecting a function, while println (of the Predef object which has been imported automatically) is a method, not a function, but as mentioned above Scala will convert it into a function automatically because a function is expected. So it works.

The second one is trying to create an anonymous function:

List(2, 3, 5) foreach ((x)=>println(x))

However, in Scala only a “top level” expression can be a function. In this case, println(_) is only a “term” in an expression (foreach is the operator) but not a top level expression, so the compiler won’t try to make it an anonymous function. Instead, it will search further to find the first enclosing top level expression (in this case, the whole expression you entered) and turn it into an anonymous function:

(x)=>List(2, 3, 5) foreach println(x)

But then the type of x can’t be inferred, so it is an error. Also, println(x) returns a value of (), the only value of the class Unit, which is not what foreach wants anyway (a function taking an Int).

With this knowledge, you can see why the third expression works:

List(2, 3, 5) foreach (println(_)) //works

This is because with the parentheses, a top level expression is expected, so Scala will make println(_) an anonymous function:

List(2, 3, 5) foreach ((x)=>println(x)) //works

Why some of expressions below work but the other don’t?

List(2, 3, 5) foreach (println("hi"+_))  //doesn't works
List(2, 3, 5) foreach (println "hi"+_)  //doesn't works
List(2, 3, 5) foreach (Predef println "hi"+_)  //works

The first expression doesn’t work because the first top level expression found is “hi”+_ due to the parentheses. So, Scala will treat it as:

List(2, 3, 5) foreach (println((x)=>"hi"+x))  //doesn't works

So you’re printing a function to the console and returning a unit value () to foreach. In order to fix the problem, you may try to get rid of the parentheses so “hi”+_ is no longer a top level expression:

List(2, 3, 5) foreach (println "hi"+_)

The problem is that Scala will now try to parse:

println "hi"+_

as:

expr1 op1 expr2 op2 ...

println is assumed to be an expression instead of a prefix operator because only !, ~, +, – can be prefix operators. So, println will be treated as an expression while the String “hi” will be treated as an operator, which is obviously incorrect.

To fix this problem, you can provide a real object as expr1, which is the Predef object:

List(2, 3, 5) foreach (Predef println "hi"+_)

Note that there are two operators: println and +. Because all symbolic operators have higher precedence than identifier operators, + will be applied first.

So, the first enclosing top level expression is turned into an anonymous function:

List(2, 3, 5) foreach ((x)=>Predef println "hi"+x)

Because in Scala “e1 op e2″ is treated as e1.op(e2), the code is treated as:

List(2, 3, 5) foreach ((x)=>Predef.println("hi".+(x)))

Introduction

This is the fifth exercise in my Scala exercises series. If you haven’t seen it before, you may want to start from the previous exercises. Below is exercise 5: monitoring pattern.

Suppose that you have created a web application displaying product information to and taking order from customers. From your testing, let’s say you have determined that it can at most display the product page 100 times per minute, otherwise it will get painfully slow or even crash. Now, you’d like to monitor the application in production to make sure its loading is not approaching that limit. If yes, you’ll promptly allocate more computing resources to it.

In order to do that, it would be great if the application would support something like SNMP and provide a variable named “display counter” for you to access. Then you could use a monitoring system like zabbix or nagios to monitor it. The good news is, there is something similar for Java: it is called JMX (Java Management eXtension).

Next, you’ll do it step by step. Your job is to fill in the missing code.

First, create a Maven project. Use the following pom.xml file:

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <groupId>scala-jmx</groupId>
    <artifactId>scala-jmx</artifactId>
    <version>1.0</version>
    <packaging>war</packaging>
    <properties>
        <spring.version>3.0.5.RELEASE</spring.version>
    </properties>
    <repositories>
        <repository>
            <id>java.net2</id>
            <name>Repository hosting the jee6 artifacts</name>
            <url>http://download.java.net/maven/2</url>
        </repository>
        <repository>
            <id>scala-tools.org</id>
            <name>Scala-tools Maven2 Repository</name>
            <url>http://scala-tools.org/repo-releases</url>
        </repository>
    </repositories>
    <pluginRepositories>
        <pluginRepository>
            <id>scala-tools.org</id>
            <name>Scala-tools Maven2 Repository</name>
            <url>http://scala-tools.org/repo-releases</url>
        </pluginRepository>
    </pluginRepositories>
    <dependencyManagement>
        <dependencies>
            <dependency>
                <groupId>org.springframework</groupId>
                <artifactId>spring-context</artifactId>
                <version>${spring.version}</version>
            </dependency>
            <dependency>
                <groupId>org.springframework</groupId>
                <artifactId>spring-core</artifactId>
                <version>${spring.version}</version>
            </dependency>
        </dependencies>
    </dependencyManagement>
    <dependencies>
        <dependency>
            <groupId>javax</groupId>
            <artifactId>javaee-web-api</artifactId>
            <version>6.0</version>
            <scope>provided</scope>
        </dependency>
        <dependency>
            <groupId>org.scala-lang</groupId>
            <artifactId>scala-library</artifactId>
            <version>2.8.0</version>
        </dependency>
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-core</artifactId>
            <version>${spring.version}</version>
        </dependency>
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-web</artifactId>
            <version>${spring.version}</version>
        </dependency>
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-context</artifactId>
            <version>${spring.version}</version>
        </dependency>
    </dependencies>
    <build>
        <pluginManagement>
            <plugins>
                <plugin>
                    <groupId>org.apache.maven.plugins</groupId>
                    <artifactId>maven-compiler-plugin</artifactId>
                    <version>2.0.2</version>
                </plugin>
                <plugin>
                    <groupId>org.scala-tools</groupId>
                    <artifactId>maven-scala-plugin</artifactId>
                    <version>2.9.1</version>
                </plugin>
            </plugins>
        </pluginManagement>
        <plugins>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-compiler-plugin</artifactId>
                <executions>
                    <execution>
                        <phase>compile</phase>
                        <goals>
                            <goal>compile</goal>
                        </goals>
                    </execution>
                </executions>
                <configuration>
                    <source>1.6</source>
                    <target>1.6</target>
                </configuration>
            </plugin>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-war-plugin</artifactId>
                <version>2.1-beta-1</version>
                <configuration>
                    <failOnMissingWebXml>false</failOnMissingWebXml>
                </configuration>
            </plugin>
            <plugin>
                <groupId>org.scala-tools</groupId>
                <artifactId>maven-scala-plugin</artifactId>
                <executions>
                    <execution>
                        <id>scala-compile-first</id>
                        <phase>process-resources</phase>
                        <goals>
                            <goal>add-source</goal>
                            <goal>compile</goal>
                        </goals>
                    </execution>
                    <execution>
                        <id>scala-test-compile</id>
                        <phase>process-test-resources</phase>
                        <goals>
                            <goal>testCompile</goal>
                        </goals>
                    </execution>
                </executions>
            </plugin>
        </plugins>
    </build>
</project>

There is nothing special except that you’ll use Spring. Next, create a Scala trait PerfCountersMBean in the com.foo package:

trait PerfCountersMBean {
  def getDisplayCounter(): Int
}

It says that you’ll have an “MBean” that has a property named displayCounter. MBean stands for managed bean, which is just a Java bean that can be inspected or called from a remote management console. Next, create a Scala class to implement the MBean in the com.foo package (fill in the code later):

...
class PerfCounters extends PerfCountersMBean {
  ...
}

In order to initialize the MBean, you’ll turn it into a Spring bean (you’ll do it later by annotating the PerfCounters class). Then, to register it with the JVM, you’ll create a Spring bean that performs the registration. So, create a beans.xml file in the src/main/resources folder:

<?xml version="1.0" encoding="UTF-8" ?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:context="http://www.springframework.org/schema/context"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xsi:schemaLocation="http://www.springframework.org/schema/beans

http://www.springframework.org/schema/beans/spring-beans-3.0.xsd

http://www.springframework.org/schema/context

http://www.springframework.org/schema/context/spring-context-3.0.xsd">

    <context:component-scan base-package="com.foo"/>
    <!-- init this exporter eagerly as nobody will ask it to export the MBeans -->
    <bean id="mbeansExporter" class="org.springframework.jmx.export.MBeanExporter" lazy-init="false">
        <property name="beans">
            <map>
                <!--
                   the key is the name of the MBean, which starts with the domain
                   name (foo.com) and then some name=value pairs.

                   the value is the Spring bean acting as the MBean
                -->
                <entry key="com.foo:name=performance-counters" value-ref="perfCounters"/>
            </map>
        </property>
    </bean>
</beans>

Of course, you need to increment the counter whenever the product page is displayed. So, create a servlet which should display something as the product information and more importantly, get access to your MBean (a Spring bean) and increment the counter (you’ll fill in the code later):

class DisplayServlet extends HttpServlet {
  ...
}

Finally, create the web.xml file the src/main/webapp/WEB-INF folder:

<?xml version="1.0" encoding="UTF-8"?>
<web-app version="3.0" xmlns="http://java.sun.com/xml/ns/javaee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd">
    <servlet>
        <servlet-name>s1</servlet-name>
        <servlet-class>com.foo.DisplayServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>s1</servlet-name>
        <url-pattern>/product</url-pattern>
    </servlet-mapping>
    <context-param>
        <param-name>contextConfigLocation</param-name>
        <param-value>classpath:/beans.xml</param-value>
    </context-param>
    <!-- initialize Spring -->
    <listener>
        <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
    </listener>
</web-app>

Run “mvn package” to build the war file. Then deploy it into Tomcat. To enable JMX in the JVM on which Tomcat runs, you need to pass some JVM arguments by setting the CATALINA_OPTS environment variable before running startup.sh (or startup.bat). On Linux:

export CATALINA_OPTS="-Dcom.sun.management.jmxremote -Dcom.sun.management.jmxremote.port=1234 -Dcom.sun.management.jmxremote.ssl=false -Dcom.sun.management.jmxremote.authenticate=false"

On Windows:

set CATALINA_OPTS="-Dcom.sun.management.jmxremote -Dcom.sun.management.jmxremote.port=1234 -Dcom.sun.management.jmxremote.ssl=false -Dcom.sun.management.jmxremote.authenticate=false"

Now, fill in all the missing code above.

To verify that it is working, run Tomcat with startup.sh (or startup.bat). Use netstat to check if it is listening on port 1234 (the port you specified above). Finally, run jconsole and connect to localhost:1234. Choose the “MBeans” tab and you should see your MBean in the com.foo folder. Check the value of the DisplayCounter. Access the product page at http://localhost:8080/scala-jmx/product. Then check the value again and it should have been incremented.

See the answer here.

package com.foo

import org.springframework.stereotype.Component
import reflect.BeanProperty
import actors.Actor

//make it a Spring component
@Component
//it will be accessed by concurrent threads, so it should be an actor
class PerfCounters extends PerfCountersMBean with Actor {
  //this will create the Java beans getter and setter
  @BeanProperty
  var displayCounter = 0

  start //start myself just like a thread

  def act() {
    while (true) {
      receive {
        //note that there is no ++ in Scala
        case IncDisplayCounterMsg => displayCounter += 1
      }
    }
  }
}

//only one instance is needed, so use a case object as the message
case object IncDisplayCounterMsg

package com.foo

import javax.servlet.http.{HttpServletResponse, HttpServletRequest, HttpServlet}
import org.springframework.web.context.support.WebApplicationContextUtils
import org.springframework.web.context.WebApplicationContext

class DisplayServlet extends HttpServlet {
  //look up the Spring application context created by the Spring context listener.
  //You can't do that in the constructor as the servlet context hasn't been set
  //yet. So, use the "lazy" keyword to do it on-demand.
  lazy val context: WebApplicationContext = WebApplicationContextUtils.getWebApplicationContext(getServletContext())

  override def doGet(req: HttpServletRequest, resp: HttpServletResponse) = {
    val writer = resp.getWriter
    writer.print("This is the product page!")
    //get access to the MBean
    val counters = context.getBean("perfCounters").asInstanceOf[PerfCounters]
    //increment the display counter by sending it a message
    counters ! IncDisplayCounterMsg
  }
}