  We study possible spin liquids on square lattice that respect all lattice
symmetries and time-reversal symmetry within the framework of Schwinger boson
(mean-field) theory. Such spin liquids have spin gap and emergent Z_2 gauge
field excitations. We classify them by the projective symmetry group method,
and find six spin liquid states that are potentially relevant to the J_1-J_2
Heisenberg model. The properties of these states are studied under mean-field
approximation. Interestingly we find a spin liquid state that can go through
continuous phase transitions to either the N\'eel magnetic order or magnetic
orders of the wavevector at Brillouin zone edge center. We also discuss the
connection between our results and the Abrikosov fermion spin liquids.
