1. In $\triangle A B C$, $O$ is the circumcenter of $\triangle A B C$, and it satisfies $\overrightarrow{A O} \cdot \overrightarrow{A B}+\overrightarrow{B O} \cdot \overrightarrow{B C}=\overrightarrow{C O} \cdot \overrightarrow{C A}$. Then the size of $\angle B$ is . $\qquad$