The default temporary-file directory is specified by the system property java.io.tmpdir. On UNIX systems the default value of this property is typically “/tmp” or “/var/tmp”; on Microsoft Windows systems it is typically “C:\WINNT\TEMP”.
JVMの設定でtmpのディレクトリを決めることができるそうで。
configuration - Environment variable to control java.io.tmpdir? - Stack Overflow
