Spark2.X 使用累加器AccumulatorV2实现字符串拼接下的字母统计

    Spark 2.X 中的累加器和 Spark 1.X中有着很大不同，下面将实现的功能是：

将一个集合，集合中含有字母 "A","B","A","D","E","D","G","H","I","A","B","I","G","D","I" ，目的是统计这个集合中字母的个数，并将其拼接成一个字符串：A=3|B=2|C=0|D=3|E=1|F=0|G=2|H=1|I=3

1、先准备拆分字符串的工具类StringUtils，主要用到了两个方法：

/**
 * 从拼接的字符串中提取字段
 * @param str 字符串
 * @param delimiter 分隔符 
 * @param field 字段
 * @return 字段值
 */
public static String getFieldFromConcatString(String str, 
      String delimiter, String field) {
   try {
      String[] fields = str.split(delimiter);
      for(String concatField : fields) {
         if(concatField.split("=").length == 2) {
            String fieldName = concatField.split("=")[0];
            String fieldValue = concatField.split("=")[1];
            if(fieldName.equals(field)) {
               return fieldValue;
            }
         }
      }
   } catch (Exception e) {
      e.printStackTrace();
   }
   return null;
}

/**
 * 从拼接的字符串中给字段设置值
 * @param str 字符串
 * @param delimiter 分隔符 
 * @param field 字段名
 * @param newFieldValue 新的field值
 * @return 字段值
 */
public static String setFieldInConcatString(String str, 
      String delimiter, String field, String newFieldValue) {
   String[] fields = str.split(delimiter);
   
   for(int i = 0; i < fields.length; i++) {
      String fieldName = fields[i].split("=")[0];
      if(fieldName.equals(field)) {
         String concatField = fieldName + "=" + newFieldValue;
         fields[i] = concatField;
         break;
      }
   }
   
   StringBuffer buffer = new StringBuffer("");
   for(int i = 0; i < fields.length; i++) {
      buffer.append(fields[i]);
      if(i < fields.length - 1) {
         buffer.append("|");  
      }
   }
   
   return buffer.toString();
}

2、实现自定义的AccumulatorV2.

import org.apache.spark.util.AccumulatorV2;

public class countAcc extends AccumulatorV2, String>{

    //定义要拼接成的字符串的格式
    String str="A=0|B=0|C=0|D=0|E=0|F=0|G=0|H=0|I=0";
    
    //Returns if this accumulator is zero value or not.
    // e.g. for a counter accumulator, 0 is zero value; for a list accumulator, Nil is zero value.
    //如果这个累加器返回值为0，可以设置为false，否则会报错 java.lang.AssertionError: assertion failed: copyAndReset must return a zero value copy
    public boolean isZero() {
        return str=="A=0|B=0|C=0|D=0|E=0|F=0|G=0|H=0|I=0";
        //return true;
    }

    //拷贝这个累加器
    public AccumulatorV2, String> copy() {
        countAcc newAccumulator = new countAcc();
        newAccumulator.str = this.str;
        return newAccumulator;
    }

    //Resets this accumulator, which is zero value. i.e. call isZero must return true.
    public void reset() {
        str="A=0|B=0|C=0|D=0|E=0|F=0|G=0|H=0|I=0";
    }

    //Takes the inputs and accumulates.
    //参数v是每次遍历RDD传进来的值，相当于每次传进来一个字母
    //在这里对字母进行拆分统计，从str字符串中找到传进来(v)的字母，找到它的value值加上1，比如：
    //str="A=0|B=0|C=0|D=0|E=0|F=0|G=0|H=0|I=0"，如果传进来的是字母A，那么将得到的新字符串为str="A=1|B=0|C=0|D=0|E=0|F=0|G=0|H=0|I=0"
    //这里因为有三个分区，将会得到
    // str1="A=2|B=1|C=0|D=1|E=1|F=0|G=0|H=0|I=0", str2="A=1|B=0|C=0|D=1|E=0|F=0|G=1|H=1|I=1", str3="A=0|B=1|C=0|D=1|E=0|F=0|G=1|H=0|I=2"
    //这三个中间结果会在下面的方法Merge中进行合并
    public void add(String v) {
        String oldValues = StringUtils.getFieldFromConcatString(str, "\\|", v);
        int newValues = Integer.valueOf(oldValues) + 1;
        String newString = StringUtils.setFieldInConcatString(str, "\\|", v, String.valueOf(newValues));
        str = newString;
    }

    //Merges another same-type accumulator into this one and update its state, i.e. this should be merge-in-place.
    //这里边other代表的是另外的相同类型的累加器，这里进行的是每个累加器的合并，相当于对上面三个str进行合并
    public void merge(AccumulatorV2, String> other) {
        countAcc o =(countAcc)other;
        String[] words = str.split("\\|");
        String[] owords = o.str.split("\\|");
        for (int i = 0; i < words.length; i++) {
            for (int j = 0; j < owords.length; j++) {
                if (words[i].split("=")[0].equals(owords[j].split("=")[0])){
                    int value = Integer.valueOf(words[i].split("=")[1]) +Integer.valueOf(owords[j].split("=")[1]);
                    String ns = StringUtils.setFieldInConcatString(str, "\\|", owords[j].split("=")[0], String.valueOf(value));
                    //每次合并完，更新str
                    str = ns;
                }
            }
        }
    }
    //Defines the current value of this accumulator
    //当前的accumulator的值
    public String value() {
        return str;
    }
}

3、测试类：

import org.apache.spark.SparkConf;
import org.apache.spark.api.java.JavaRDD;
import org.apache.spark.api.java.JavaSparkContext;
import org.apache.spark.api.java.function.VoidFunction;
import java.util.Arrays;
import java.util.List;
public class countAccumulator {
    public static void main(String[] args) {
        SparkConf conf = new SparkConf().setAppName("AccumulatorTest").setMaster("local");
        JavaSparkContext sc = new JavaSparkContext(conf);
        List list = Arrays.asList("A","B","A","D","E","D","G","H","I","A","B","I","G","D","I");
        final JavaRDD javaRDD = sc.parallelize(list, 3).cache();
        final countAcc sa = new countAcc();
        sc.sc().register(sa,"sa");
        javaRDD.foreach(new VoidFunction() {
            public void call(String s) throws Exception {
                sa.add(s);
            }
        });
        System.out.println(sa.value());
    }
}

可以得到结果：A=3|B=2|C=0|D=3|E=1|F=0|G=2|H=1|I=3