如何修复mapreduce中mapper的setup方法给出的字符串值的不规则行为（） _MapReduce

出门莫恨无人随，书中车马多如簇。这篇文章主要讲述如何修复mapreduce中mapper的setup方法给出的字符串值的不规则行为？相关的知识，希望能为你提供帮助。
【如何修复mapreduce中mapper的setup方法给出的字符串值的不规则行为（）】我是MapReduce的新手，并且正在学习设置方法的实现。配置给出的新字符串值正确打印，但是当我尝试进一步处理它时，字符串的初始值就会生效。我知道字符串是不可变的，但它应该提供当前指向其他方法的值。

public class EMapper extends Mapper< LongWritable, Text, Text, Text> {String wordstring = "abcd"; //initialized wordstring with "abcd"public void setup(Context context) { Configuration config = new Configuration(context.getConfiguration()); wordstring = config.get("mapper.word"); // As string is immutable, // wordstring should now point to // value given by mapper.word //Here mapper.word="ankit" by //using -D in hadoop command}String def = wordstring; String jkl = String.valueOf(wordstring); //tried to copy current value //but //string jkl prints the initial /value.public void map(LongWritable key, Text value, Context context) throws InterruptedException, IOException { context.write(new Text("wordstring=" + wordstring + "" + "def=" + def), new Text("jkl=" + jkl)); } }public class EDriver extends Configured implements Tool {private static Logger logger = LoggerFactory.getLogger(EDriver.class); public static void main(String[] args) throws Exception { logger.info("Driver started"); int res = ToolRunner.run(new Configuration(), new EDriver(), args); System.exit(res); }public int run(String[] args) throws Exception { if (args.length != 2) { System.err.printf("Usage: %sneedsarguments", getClass().getSimpleName()); return -1; } Configuration conf = getConf(); Job job = new Job(conf); job.setJarByClass(EDriver.class); job.setJobName("E Record Reader"); job.setMapperClass(EMapper.class); job.setMapOutputKeyClass(Text.class); job.setMapOutputValueClass(Text.class); job.setReducerClass(EReducer.class); job.setNumReduceTasks(0); job.setOutputKeyClass(Text.class); job.setOutputValueClass(NullWritable.class); FileInputFormat.addInputPath(job, new Path(args[0])); FileOutputFormat.setOutputPath(job, new Path(args[1])); job.setInputFormatClass(ExcelInputFormat.class); return job.waitForCompletion(true) ? 0 : 1; }}

我期待输出

wordstring=ankitdef=ankitjkl=ankit

实际输出是

wordstring=ankitdef=abcdjkl=abcd

答案这与字符串的可变性无关，而与代码执行顺序有关。
只有在执行任何类级别命令后才会调用setup方法。您编写代码的顺序不会改变任何内容。如果您按照实际执行的顺序重新编写代码的顶部，则可以：

public class EMapper extends Mapper< LongWritable, Text, Text, Text> { String wordstring = "abcd"; String jkl = String.valueOf(wordstring); public void setup(Context context) { Configuration config = new Configuration(context.getConfiguration()); wordstring = config.get("mapper.word"); //By the time this is called, jkl has already been assigned to "abcd" }

因此，jkl仍然是abcd并不奇怪。您应该在jkl方法中设置setup，如下所示：

public class EMapper extends Mapper< LongWritable, Text, Text, Text> { String wordstring; String jkl; public void setup(Context context) { Configuration config = new Configuration(context.getConfiguration()); wordstring = config.get("mapper.word"); jkl = wordstring; //Here, jkl and wordstring are both different variables pointing to "ankit" }//Here, jkl and wordstring are null, as setup(Context context) has not yet runpublic void map(LongWritable key, Text value, Context context) throws InterruptedException, IOException { //Here, jkl and wordstring are both different variables pointing to "ankit" context.write(new Text("wordstring=" + wordstring), new Text("jkl=" + jkl)); }

当然你实际上并不需要jkl，你可以直接使用wordstring。
另一答案问题已经解决了。实际上，我在分布式模式下运行Hadoop，其中SETUP，MAPPER，REDUCER和CLEANUP在不同的JVM上运行。因此，数据无法直接从SETUP传输到MAPPER。第一个wordstring对象在mapper中被初始化为“abcd”。我试图改变SETUP中的wordstring（创建了wordstring的另一个对象），这实际上是在另一个JVM中发生的。所以，当我试图复制jkl中的“wordstring”时
String jkl = String.valueOf（wordstring）;
wordstring的第一个值（由mapper创建并初始化为“abcd”）被复制到jkl。
如果我在独立模式下运行Hadoop，它将使用单个JVM，并且SETUP给予wordstring的值将被复制到jkl。
因此，jkl将wordstring的副本初始化为“abcd”而不是SETUP给出的副本。
我用了
HashMap map = new HashMap（）;
在SETUP到MAPPER之间传输数据，然后jkl获得了SETUP的字符串给出的值的副本。