IBM User Guide for Java V7 on Windows > Troubleshooting and support > Using diagnostic tools > Using Heapdump > Portable Heap Dump (PHD) file format

PHD object records

PHD files can contain short, medium, and long object records, depending on the number of object references in the Heapdump.

Short object record

The short object record includes detailed information within the tag "byte". This information includes:

• The 1 byte tag. The top bit (0x80) is set and the following 7 bits in descending order contain:
  • 2 bits for the class cache index. The value represents an index into a cache of the last four classes used.
  • 2 bits containing the number of references. Most objects contain 0 - 3 references. If there are 4 - 7 references, the medium object record is used. If there are more than seven references, the long object record is used.
  • 1 bit to indicate whether the gap is a "byte" or a "short". The gap is the difference between the address of this object and the previous object. If set, the gap is a "short". If the gap does not fit into a "short", the "long" object record form is used.
  • 2 bits indicating the size of each reference. The following values apply:
    • 0 indicates "byte" format.
    • 1 indicates "short" format.
    • 2 indicates "integer" format.
    • 3 indicates "long" format.

• A "byte" or a "short" containing the gap between the address of this object and the address of the preceding object. The value is signed and represents the number of 32-bit "words" between the two addresses. Most gaps fit into 1 byte.
• If all objects are hashed, a "short" containing the hashcode.
• The array of references, if references exist. The tag shows the number of elements, and the size of each element. The value in each element is the gap between the address of the references and the address of the current object. The value is a signed number of 32-bit "words". Null references are not included.

Medium object record

These records provide the actual address of the class rather than a cache index. The format is:

• The 1 byte tag. The second bit (0x40) is set and the following 6 bits in descending order contain:
  • 3 bits containing the number of references.
  • 1 bit to indicate whether the gap is a 1 byte value or a "short" For more information, see the description in the short record format.
  • 2 bits indicating the size of each reference. For more information, see the description in the short record format.

• A "byte" or a "short" containing the gap between the address of this object and the address of the preceding object. For more information, see the description in the short record format.
• A "word" containing the address of the class of this object.
• If all objects are hashed, a "short" containing the hashcode.
• The array of references. For more information, see the description in the short record format.

Long object record

This record format is used when there are more than seven references, or if there are extra flags or a hashcode. The record format is:

• The 1 byte tag, containing the value 4.
• A "byte" containing flags, with these bits in descending order:
  • 2 bits to indicate whether the gap is a "byte", "short", "int" or "long" format.
  • 2 bits indicating the size of each reference. For more information, see the description in the short record format.
  • 2 unused bits.
  • 1 bit indicating if the object was hashed and moved. If this bit is set then the record includes the hashcode.
  • 1 bit indicating if the object was hashed.

• A "byte", "short", "int" or "long" containing the gap between the address of this object and the address of the preceding object. For more information, see the description in the short record format.
• A "word" containing the address of the class of this object.
• If all objects are hashed, a "short" containing the hashcode. Otherwise, an optional "int" containing the hashcode if the hashed and moved bit is set in the record flag byte.
• An "int" containing the length of the array of references.
• The array of references. For more information, see the description in the short record format.

Parent: Portable Heap Dump (PHD) file format

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