  How quarks and gluons conspire to provide the total spin of proton is a
long-standing puzzle in quantum chromodynamics (QCD). The unique capability of
RHIC, that can provide longitudinally polarized $p+p$ collisions at both
$\sqrt{s} = 200$ GeV and $\sqrt{s} = 510$ GeV, opened new territory to
constrain the helicity structure of the proton with unprecedented depth and
precision.
  Results from various STAR spin measurements have contributed significantly to
our understanding of the quark and gluon helicity distributions inside the
proton. The longitudinal double-spin asymmetry, $A_{LL}$, from the STAR 2009
inclusive jet measurement, provides the first indication of the positive gluon
polarization with partonic momentum fraction $x$ greater than 0.05 inside the
proton. More precise measurements using the $p+p$ data collected in 2012, 2013
and 2015 at both $\sqrt{s}$ = 510 and 200 GeV confirm the previous findings and
provide additional constraints in the largely unexplored region of $x < 0.05$.
Compared to the inclusive jet observables, analyses of dijet production
extending to higher pseudorapidity (up to $\eta \sim 1.8$) provide better
constraints on the $x$ dependent behavior of $\Delta g(x)$. Moreover, the
reconstruction of $W^{\pm}$ in longitudinally polarized proton-proton
collisions provides significant constraints on the flavor separation of the
light sea quark helicity distributions inside the proton, while the
longitudinal spin transfer to $\Lambda$ and $\bar \Lambda$ hyperons provides
access to the helicity of strange and anti-strange quarks in the proton.
  In this proceeding, an overview of dijet measurements from longitudinally
polarized proton-proton collisions at STAR is presented.
