Anyone who visits Java cannot fail to be impressed by the string of volcanoes which dominate the skyline of much of the island. I finally got round to climbing the first of the big ones in 2009 and before long I realised that with a little bit of careful planning I could get to the top of all 12 of the 3000m peaks marked on the Nelles map of Java
Although no special equipment is required other than a good pair of shoes, a couple of torches with spare batteries (and a small kerosene stove for the longer visits), these are serious mountains and every year a number of Indonesians collapse and die on them; some wander off the paths, while others are not prepared for the cold nights where the temperature may fall to near freezing near the summits. For practical reasons, it makes sense to undertake expeditions all the peaks tend to cloud up during the day and the best time to reach a summit is probably at dawn - personally I do not enjoy climbing in the dark as many of the trails are in poor condition owing to overuse. The question of access/permission/registration to climb is a very grey area, in some cases, you very definitely need an official permit, in other cases you can just pitch up and go. For most of the volcanoes, there is a point at which you can register your presence and this obviously makes good sense from a safety point of view.
In some cases you would be wise to take a local guide (usually these can be recruited on the spot for U$10 a day or so), for others a small group of 2 or 3 can climb independently, especially the more popular peaks where there are always crowds of Indonesian students (and their rubbish). NEVER go out on one of these mountains alone and always take sufficient water - in just a few cases water is available on the climb. Instant noodles are a staple of local climbers while it makes sense to take along high energy food items such as nuts, chocolate and bread.