10% of the world's active volcanoes reside in Japan, more than any other country, 100 of which are currently active.  These volcanoes are part of the Pacific Ring of Fire and are caused by subduction zones of the Pacific plate beneath continental and other oceanic plates along its margins.

The Japan Meteorological Agency (JMA) provides the nation with early warning information for earthquakes, tsunamis and volcanic eruptions.  Usually the general population will get a decent warning before any threatening volcanic activity since magma flow is easier to predict than an earthquake but of course volcanoes are not 100% predictable and can erupt without warning.  
Which is what happened on September 27, 2014 at the second largest volcano in Japan, Mount Ontake, when volcano classified as a level 1 at the time(normal/nonthreatening) erupted without magma.  At least 56 hikers were killed and many more injured and missing.  In rare cases like this when no prior warning is given there is little that can be down to take precautionary measures.  

Mount Ontake during it's eruption on September 27, 2014

Some of active volcanoes in Japan have now be equipped with reinforced concrete shelters near the crater and along trails for hikers to take shelter in during a sudden eruption.  After the devastation at Mount Ontake which many believe could have been lessened with an adequate amount of shelters the nation want's to increase the amount provided at each volcano but budgeting is currently an issue.  Currently Mount Fuji one of Japan's most popular hiking spots has no shelters in place.  

Concrete shelters

V.Grasso and A.Singh, UNEP (Dec 2011, forthcoming): Early Warning Systems: www.jma.go.jp/jm/en/Activities/earthquake.html,.com

"Volcanoes of Japan" Volcano Discovery.  http://www.volcanodiscovery.com/