conscription does not exist
According to art. 9 of the 1947 Constitution Japan is not allowed to have armed forces: "Aspiring sincerely to an international peace based on justice and order, the Japanese people forever renounce war as a sovereign right of the nation and the threat or use of force as means of settling international disputes. In order to accomplish the aim of the preceding paragraph, land sea and air forces, as well as other war potential, will never be maintained. The right of belligerency of the state will not be recognized."
This clause in the constitution was prompted by the allied forces' determination after the second world war, to disarm, demilitarize and democratize Japan so that it would never again be a threat to peace.
Despite the clause in the constitution, Japan does in fact have armed forces: the Jietitai - Self-Defence Force (SDF) - which was established in 1954. The SDF was initially referred to in public as the paramilitary or para-police forces, but in 1980 Prime Minister Suzuki publicly described it as a military force. 
The existence of the SDF has been the subject of continuous legal and political debate in Japan since the 1950s. The socialist opposition has initiated several court cases in the Supreme Court concerning the constitutionality of the SDF. However the Supreme Court has always ruled that the SDF was constitutional.  
The Liberal Democratic Party government has made several unsuccessful attempts to amend art. 9 of the constitution, their failure being due to their inability to get the two-thirds majority necessary for a constitutional amendment. 
The SDF has difficulties over obtaining the requisite number of recruits. Re-introduction of conscription has never been considered, being deemed to be forbidden under art. 18 of the Constitution, which forbids involuntary servitude. 
Legal recruitment age for the SDF is 18, and 15 for SDF youth cadets. Youth cadets are trained to become technical specialists within the SDF, their education including basic military training. 
2 Conscientious objection
There is no known legal provision for conscientious objection.
It seems that professional soldiers who develop a conscientious objection may seek discharge at any time (see below).
Japan has no distinct military legislation as courts martial are forbidden under art. 76 of the Constitution. This means that SDF personnel can resign their commissions at any time. SDF personnel charged with military offences are entitled to see relevant military documents and to have a civilian defence. 
Conscription had been introduced in Japan as early as 1872. Initially designed for domestic security, the armed forces inherited the prestige formerly accorded to the samurai and became an instrument of national unity. The armed forces grew significantly in the 1920s, and by 1940 they comprised more than five million soldiers. Conscription ended in 1945 when Japan surrendered to the allied forces. 
6 Annual statistics
The SDF comprise 235,600 troops, that is 0.19 percent of the population. 
 Japanese Embassy in the Netherlands 1996. Response to CONCODOC questionnaire, The Hague, 12 December 1996.  Renwick, Neil 1996. Japan's Alliance Politics and Defence Production. MacMillan, Oxford.  Bowring, Richard and Peter Kornicki (ed.) 1993. The Cambridge Encyclopedia of Japan. Cambridge University Press, Cambridge.  Hunter, Janet (ed.) 1984. Concise Dictionary of Modern Japanese History. University of California Press, Berkeley/Los Angeles/London.  Japanese permanent mission to the United Nations 1998. Response to Quaker United Nations Office inquiry. Geneva, 1998.  Institute for Strategic Studies 1997. Military Balance 1997/98. ISS, London.