ObjectIn/OutputStream improvements

[Robert Stupp]

Hi,

I digged through the object serialization code and found some lines that could be optimized to reduce the number of calls to System.arraycopy() and temporary object allocations especially during string (de)serialization.
In short sentences the changes are:
ObjectInputStream:
- skip primitive/object reading if no primitive/objects in class (defaultReadFields method)
- use shared StringBuilder for string reading (prevent superfluous object allocations of one StingBuilder and one implicit char[] for each string being deserialized)
ObjectOutputStream:
- skip primitive/object writing if no primitive/objects in class (defaultWriteFields method)
- use unsafe access to calculate UTF-length
- use unsafe access in readBytes() and writeChars() methods to access String value field
- removed cbuf field
ObjectStreamClass/ObjectStreamField:
- minor improvement in getClassSignature ; share method code with ObjectStreamField (return constant string for primitives)

I have tested the changes in a big Java installation at a customer (backported the Java8 ObjectIn/OutputStream including the changes to Java6) and a long running real application performance test resulted in reduced CPU usage (from about 60% per server to 50%).
The changes I made in openjdk8 pass all tests.

Since I have no experience how to contribute code to openjdk in form of a push/changeset I have added the diff (hg export -g) to this email.

Robert