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 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 pi 5 10

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.plugins</groupId>
     <artifactId>maven-jar-plugin</artifactId>
      <configuration>
       <archive>
         <manifest>
           <mainClass>org.apache.hadoop.examples.ExampleDriver</mainClass>
         </manifest>
       </archive>
     </configuration>
    </plugin>

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

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

  /**
   * A description of a program based on its class and a 
   * human-readable description.
   */
  Map<String, ProgramDescription> programs;

  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;
  }

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。

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