I'm using now netbeans 6.0m1 to work with glassfish sources. I found the following things really useful:
1. It is very easy to integrate glassfish - even afterwards. I installed glassfish standalone and integrated it afterwards. It was really easy.
Actually I do not like a very tight integration between IDE and appserver, because of possible side effects. In this case it seems like netbeans controls glassfish from outside.
2. You can also manage glassfish from netbeans. It is possible to browse JDBC datasources, pools etc. Only the detailed view could be improved (key-value pair view).
3. It is also very easy to connect with database and browse the tables. This functionality is similar with eclipse's DTP or plugins like quantumdb, but leaner.
4. Netbeans 6.0m1 seems to be very stable. I run it since 2 weeks non-stop (I'm working on a laptop and I use the hibernate mode instead of shut down.), without any crashes, memory leaks etc.
5. Glassfish consists of about 70.000 files (I know it, because WinZip refused to zip the project, because of the internal restriction...:-)). I manage this files with netbeans without any problems.
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