org.apache.avro.io
Class ResolvingDecoder

java.lang.Object
  org.apache.avro.io.Decoder
      org.apache.avro.io.ValidatingDecoder
          org.apache.avro.io.ResolvingDecoder

public class ResolvingDecoder
extends ValidatingDecoder

Decoder that peforms type-resolution between the reader's and writer's schemas.

When resolving schemas, this class will return the values of fields in _writer's_ order, not the reader's order. (However, it returns _only_ the reader's fields, not any extra fields the writer may have written.) To help clients handle fields that appear to be coming out of order, this class defines the methods readFieldName() and readFieldIndex(). When called just before reading the value of a record-field, they return the name/index of the field about to be read. See readFieldIndex() for usage details.

See the parser documentation for information on how this works.

Field Summary

Fields inherited from class org.apache.avro.io.ValidatingDecoder

in, pos, stack

Constructor Summary

ResolvingDecoder(Schema writer, Schema reader, Decoder in)

Method Summary

protected int	advance(int input)
double	readDouble() Reads a double written by Encoder.writeDouble(double).
int	readEnum() Reads an enumeration.
int	readFieldIndex() Returns the (zero-based) index of the next field of the record we're reading.
String	readFieldName() Returns the name of the next field of the record we're reading.
int	readIndex() Reads the tag of a union written by Encoder.writeIndex(int).
long	readLong() Reads a long written by Encoder.writeLong(long).
void	reset() After reading a complete object that conforms to the schema or after an error, if you want to start reading another object, call this method.
protected int	skipSymbol(int sym, int p)

Methods inherited from class org.apache.avro.io.ValidatingDecoder

arrayNext, expand, init, mapNext, readArrayStart, readBoolean, readBytes, readFixed, readFloat, readInt, readMapStart, readNull, readString, skipArray, skipBytes, skipFixed, skipMap, skipProduction, skipString

Methods inherited from class org.apache.avro.io.Decoder

readFixed

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

ResolvingDecoder

public ResolvingDecoder(Schema writer,
                        Schema reader,
                        Decoder in)
                 throws IOException

Throws:

IOException

Method Detail

readFieldName

public String readFieldName()
                     throws IOException

Returns the name of the next field of the record we're reading. Similar to readFieldIndex() -- see that method for details.

Throws:

IllegalStateExcpetion - If we're not about to read a record-field

IOException

readFieldIndex

public int readFieldIndex()
                   throws IOException

Returns the (zero-based) index of the next field of the record we're reading. This method is useful because ResolvingDecoder returns values in the order written by the writer, rather than the order expected by the reader. This method allows reader's to figure out what fields to expect. Let's say the reader is expecting a three-field record, the first field is a long, the second a string, and the third an array. In this case, a typical usage might be as follows:

   for (int i = 0; i < 3; i++) {
     switch (in.readFieldIndex()) {
     case 1:
       foo(in.readLong());
       break;
     case 2:
       someVariable = in.readString();
       break;
     case 3:
       bar(in); // The code of "bar" will read an array-of-int
       break;
     }
 

Note that ResolvingDecoder will return only the fields expected by the reader, not other fields that may have been written by the writer. Thus, the iteration-count of "3" in the above loop will always be correct. Throws a runtime exception if we're not just about to read the field of a record. Also, this method (and readFieldName()) will consume the field information, and thus may only be called once before reading the field value. (However, if the client knows the order of incoming fields and does not need to reorder them, then the client does not need to call this or readFieldName().)

Throws:

IllegalStateExcpetion - If we're not about to read a record-field

IOException

readLong

public long readLong()
              throws IOException

Description copied from class: Decoder

Reads a long written by Encoder.writeLong(long).

Overrides:

readLong in class ValidatingDecoder

Throws:

IOException

readDouble

public double readDouble()
                  throws IOException

Description copied from class: Decoder

Reads a double written by Encoder.writeDouble(double).

Overrides:

readDouble in class ValidatingDecoder

Throws:

IOException

readEnum

public int readEnum()
             throws IOException

Description copied from class: Decoder

Reads an enumeration.

Overrides:

readEnum in class ValidatingDecoder

Returns:

The enumeration's value.

Throws:

IOException

readIndex

public int readIndex()
              throws IOException

Description copied from class: Decoder

Reads the tag of a union written by Encoder.writeIndex(int).

Overrides:

readIndex in class ValidatingDecoder

Throws:

IOException

skipSymbol

protected int skipSymbol(int sym,
                         int p)
                  throws IOException

Overrides:

skipSymbol in class ValidatingDecoder

Throws:

IOException

advance

protected int advance(int input)
               throws IOException

Overrides:

advance in class ValidatingDecoder

Throws:

IOException

reset

public void reset()
           throws IOException

Description copied from class: Decoder

After reading a complete object that conforms to the schema or after an error, if you want to start reading another object, call this method.

Overrides:

reset in class ValidatingDecoder

Throws:

IOException