In Mustang there is an VM attach mechanism that is exposed as a com.sun API. The API ships in the JDK (not the JRE). The implementation has some native methods in attach.dll/libattach.so. When the feature was putback to the j2se workspace the make/common/Release.gmk rules were updated so that the native library is copied into the j2sdk-image and not the j2re-image.

Although the images builds are correctly, it seems that attach.dll is missing from the Windows installer when the JDK is installed. For example, if I install the latest JDK from
http://www.java.net/download/jdk6/binaries/jdk-6_0-ea-bin-b45-windows-i586-28_jul_2005.exe and select the SDK bits then attach.dll is not copied into C:\Program Files\Java\jdk1.6.0\jre\bin.