Hadoop2.6.0子项目hadoop-mapreduce-examples的简单介绍

引文

学习Hadoop的同学们，一定知道如果运行Hadoop自带的各种例子，以大名鼎鼎的wordcount为例，你会输入以下命令：

hadoop org.apache.hadoop.examples.WordCount -D mapreduce.input.fileinputformat.split.maxsize=1 /wordcount/input /wordcount/output/result1

hadoop org.apache.hadoop.examples.WordCount -D mapreduce.input.fileinputformat.split.maxsize=1 /wordcount/input /wordcount/output/result1

当然，有些人还会用以下替代方式：

hadoop jar share/hadoop/mapreduce/hadoop-mapreduce-examples-2.6.0.jar wordcount /wordcount/input /wordcount/output/result1

hadoop jar share/hadoop/mapreduce/hadoop-mapreduce-examples-2.6.0.jar wordcount /wordcount/input /wordcount/output/result1

相比于原始的执行方式，使用jar命令方式，让我们不用再敲入繁琐的完整包路径。比如我们知道hadoop-mapreduce-examples项目中还提供了其它的例子，比如计算圆周率的例子，我们只需要记住此应用的简单名字pi，就可以执行它：

hadoop jar share/hadoop/mapreduce/hadoop-mapreduce-examples-2.6.0.jar pi 5 10

hadoop jar share/hadoop/mapreduce/hadoop-mapreduce-examples-2.6.0.jar pi 5 10

虽说我们只是使用这些现成的例子，没有必要较真，但是这种简洁的使用方式，无疑还是值得借鉴的。本文将分析下这种方式实现的原理，有兴趣的同学可以一读。

源码分析

这一节，我们通过对hadoop-mapreduce-examples项目中的关键源码进行分析，理解简洁执行的原理。在hadoop-mapreduce-examples项目的pom.xml文件中配置了org.apache.hadoop.examples.ExampleDriver作为jar命令的入口，配置如下：

<plugin>
 <groupId>org.apache.maven.pluginsgroupId>
  <artifactId>maven-jar-pluginartifactId>
   <configuration>
    <archive>
      <manifest>
        <mainClass>org.apache.hadoop.examples.ExampleDrivermainClass>
      manifest>
    archive>
  configuration>
 plugin>

   
    org.apache.maven.plugins
     maven-jar-plugin
      
       
         
           org.apache.hadoop.examples.ExampleDriver
         
       
     
    

这决定了使用jar命令执行hadoop-mapreduce-examples-2.6.0.jar包时，实际执行了ExampleDriver的main方法，ExampleDriver的实现如下：

public class ExampleDriver {

  public static void main(String argv[]){
    int exitCode = -1;
    ProgramDriver pgd = new ProgramDriver();
    try {
      pgd.addClass("wordcount", WordCount.class,
                   "A map/reduce program that counts the words in the input files.");
      // 省略其它例子的注册代码
      pgd.addClass("pi", QuasiMonteCarlo.class, QuasiMonteCarlo.DESCRIPTION);
      // 省略其它例子的注册代码
      exitCode = pgd.run(argv);
    }
    catch(Throwable e){
      e.printStackTrace();
    }

    System.exit(exitCode);
  }
}

public class ExampleDriver {
  
  public static void main(String argv[]){
    int exitCode = -1;
    ProgramDriver pgd = new ProgramDriver();
    try {
      pgd.addClass("wordcount", WordCount.class, 
                   "A map/reduce program that counts the words in the input files.");
      // 省略其它例子的注册代码
      pgd.addClass("pi", QuasiMonteCarlo.class, QuasiMonteCarlo.DESCRIPTION);
      // 省略其它例子的注册代码
      exitCode = pgd.run(argv);
    }
    catch(Throwable e){
      e.printStackTrace();
    }
    
    System.exit(exitCode);
  }
}

以上代码构造了ProgramDriver的实例，并且调用其addClass方法，三个参数分别是例子名称（如wordcount、pi等）、例子的实现Class、例子的描述信息。ProgramDriver的addClass方法的实现如下：

public void addClass(String name, Class mainClass, String description)
    throws Throwable {
  programs.put(name , new ProgramDescription(mainClass, description));
}

  public void addClass(String name, Class mainClass, String description)
      throws Throwable {
    programs.put(name , new ProgramDescription(mainClass, description));
  }

首先，构造ProgramDescription对象，其构造函数如下：

public ProgramDescription(Class mainClass,
                          String description)
  throws SecurityException, NoSuchMethodException {
  this.main = mainClass.getMethod("main", paramTypes);
  this.description = description;
}

    public ProgramDescription(Class mainClass, 
                              String description)
      throws SecurityException, NoSuchMethodException {
      this.main = mainClass.getMethod("main", paramTypes);
      this.description = description;
    }

其中main的类型是 java.lang.reflect.Method，用于保存例子Class的main方法。
然后，将例子名称（如wordcount、pi等）和ProgramDescription实例注册到programs中，programs的类型定义如下：

/**
 * A description of a program based on its class and a
 * human-readable description.
 */
Map programs;

  /**
   * A description of a program based on its class and a 
   * human-readable description.
   */
  Map programs;

ExampleDriver的main方法在最后会调用ProgramDriver的run方法，其实现如下：

public int run(String[] args)
  throws Throwable
{
  // Make sure they gave us a program name.
  if (args.length == 0) {
    System.out.println("An example program must be given as the" +
                       " first argument.");
    printUsage(programs);
    return -1;
  }

  // And that it is good.
  ProgramDescription pgm = programs.get(args[0]);
  if (pgm == null) {
    System.out.println("Unknown program '" + args[0] + "' chosen.");
    printUsage(programs);
    return -1;
  }

  // Remove the leading argument and call main
  String[] new_args = new String[args.length - 1];
  for(int i=1; i < args.length; ++i) {
    new_args[i-1] = args[i];
  }
  pgm.invoke(new_args);
  return 0;
}

  public int run(String[] args)
    throws Throwable 
  {
    // Make sure they gave us a program name.
    if (args.length == 0) {
      System.out.println("An example program must be given as the" + 
                         " first argument.");
      printUsage(programs);
      return -1;
    }
	
    // And that it is good.
    ProgramDescription pgm = programs.get(args[0]);
    if (pgm == null) {
      System.out.println("Unknown program '" + args[0] + "' chosen.");
      printUsage(programs);
      return -1;
    }
	
    // Remove the leading argument and call main
    String[] new_args = new String[args.length - 1];
    for(int i=1; i < args.length; ++i) {
      new_args[i-1] = args[i];
    }
    pgm.invoke(new_args);
    return 0;
  }

ProgramDriver的run方法执行的步骤如下：

1. 参数长度校验；
2. 根据第一个参数，从programs中查找对应的ProgramDescription实例；
3. 将其余的参数传递给ProgramDescription的invoke方法，进而执行对应的例子。

ProgramDescription的invoke方法的实现如下：

public void invoke(String[] args)
  throws Throwable {
  try {
    main.invoke(null, new Object[]{args});
  } catch (InvocationTargetException except) {
    throw except.getCause();
  }
}

    public void invoke(String[] args)
      throws Throwable {
      try {
        main.invoke(null, new Object[]{args});
      } catch (InvocationTargetException except) {
        throw except.getCause();
      }
    }

由此我们知道具体例子的执行，是通过反射调用具体例子Class的main方法，最终实现的。