# How to measure a qubit Hamiltonian in qiskit

I am using qiskit to get some measurement results of observables similar to the Hamiltonian. Can someone provide the way how qiskit measures the Hamiltonian (Jordan-Wigner transformed) when using VQE? Or can they just provide the source code link.

A projective measurement is described by an observable, $$M$$, a Hermitian operator on the state space of the system being observed. The observable has a spectral decomposition, $$M = \sum_{m} mP_{m}$$ where $$P_{m}$$ is the projector onto the eigenspace of $$M$$ with eigenvalue $$m$$. The possible outcomes of the measurement correspond to the eigenvalues, $$m$$, of the observable. Upon measuring the state $$|\psi\rangle$$, the probability of getting result $$m$$ is $$p(m) = \langle\psi|P_{m}|\psi\rangle$$ Given that outcome $$m$$ occurred, the state of the quantum system immediately after the measurement is $$\frac{P_{m}|\psi\rangle}{\sqrt{p(m)}}$$