Minecraft server out of memory when attempt of login in to server

When loggin into my server, my server says this in the console: [SEVERE] Encountered an unexpected exception OutOfMemoryError
java.lang.OutOfMemoryError: Java heap space
at dl.<init>(SourceFile:64)
at ih.j_(SourceFile:147)
at xe.a(SourceFile:1218)
at ie.a(SourceFile:424)
at xe.g(SourceFile:1197)
at xe.h(SourceFile:1122)
at ie.h(SourceFile:352)
at net.minecraft.server.MinecraftServer.r(SourceFile:718)
at hg.r(SourceFile:233)
at net.minecraft.server.MinecraftServer.q(SourceFile:661)
at net.minecraft.server.MinecraftServer.run(SourceFile:595)
at fp.run(SourceFile:818)

It happens every time a player logs in.