Java Hadoop problems with ChainMapper Generics

Question

嗨，我目前正面临Java泛型的问题：我需要使用的方法有这个签名

static<K1,V1,K2,V2> void addMapper(JobConf job, 
           Class<? extends Mapper<K1,V1,K2,V2>> klass, 
           Class<? extends K1> inputKeyClass, 
           Class<? extends V1> inputValueClass, 
           Class<? extends K2> outputKeyClass, 
           Class<? extends V2> outputValueClass, 
           boolean byValue, JobConf mapperConf) 

这就是我所说的

ChainMapper.addMapper(conf, NameMapper.class, 
           Object.class, Object.class, Object.class, Object.class, 
           false, nameYearConf);

其中NameMapper定义如下

public class NameMapper extends Mapper<Object, Object, Object, Object> { }

另外两个参数也正确初始化

JobConf conf = new JobConf(); JobConf nameYearConf = new JobConf(false);

当我试图编译时，我得到了以下错误

    srcMeanYear.java:107: error: method addMapper in class ChainMapper cannot be applied to given types;
                    ChainMapper.addMapper(conf, NameMapper.class, Object.class,
                               ^
      required: JobConf,Class<? extends Mapper<K1,V1,K2,V2>>,Class<? extends K1>,Class<? extends V1>,Class<? extends K2>,Class<? extends V2>,boolean,JobConf
      found: JobConf,Class<MeanYear.NameMapper>,Class<Object>,Class<Object>,Class<Object>,Class<Object>,boolean,JobConf
      reason: no instance(s) of type variable(s) K1,V1,K2,V2 exist so that argument type Class<MeanYear.NameMapper> conforms to formal parameter type Class<? extends Mapper<K1,V1,K2,V2>>
      where K1,V1,K2,V2 are type-variables:
        K1 extends Object declared in method <K1,V1,K2,V2>addMapper(JobConf,Class<? extends Mapper<K1,V1,K2,V2>>,Class<? extends K1>,Class<? extends V1>,Class<? extends K2>,Class<? extends V2>,boolean,JobConf)
        V1 extends Object declared in method <K1,V1,K2,V2>addMapper(JobConf,Class<? extends Mapper<K1,V1,K2,V2>>,Class<? extends K1>,Class<? extends V1>,Class<? extends K2>,Class<? extends V2>,boolean,JobConf)
        K2 extends Object declared in method <K1,V1,K2,V2>addMapper(JobConf,Class<? extends Mapper<K1,V1,K2,V2>>,Class<? extends K1>,Class<? extends V1>,Class<? extends K2>,Class<? extends V2>,boolean,JobConf)
        V2 extends Object declared in method <K1,V1,K2,V2>addMapper(JobConf,Class<? extends Mapper<K1,V1,K2,V2>>,Class<? extends K1>,Class<? extends V1>,Class<? extends K2>,Class<? extends V2>,boolean,JobConf)

我真的不明白我做错了什么，我尝试了一些类似的例子和编译，它们都起作用了。我也尝试过这里提供的一些例子，但没有成功。谢谢

Answer 1

乍一看，我猜你是在混合新的和旧的api包中的类：

ChainMapper（自1.1.2起）仅针对旧的API包(org.apache.hadoop.mapred）实现，尚未移植到新的API包（org.apache.hadoop.mapreduce）。

因此，ChainMapper.addMapper（Class<? extends Mapper<K1,V1,K2,V2>> klass）的第二自变量被期望为org.apache.hadoop.mapred.Mapper类型。

由于您当前的映射器NameMapper扩展了mapper类，这使我相信这是新的api mapper类（org.apache.hadoop.mapreduce），因为新的api是一个类，而不是接口

您应该能够通过修改NameMapper类的签名来修复，以匹配旧的API Mapper接口的签名：

public class NameMapper extends MapReduceBase 
                        implements Mapper<Object, Object, Object, Object> { }

这也意味着您当前地图方法的签名需要从更改

protected void map(Object key, Object value, Context context) { }
// to
public void map(Object key, Object value, OutputCollector collector,
                Reporter reporter) { }

如果你正在使用新的Mapper API的设置和清理方法，你也需要替换它们：

public void close() { } // instead of cleanup(Context) {}
public void configure(JobConf conf) { } // instead of setup(Context) {}