Three-dimensional integration is considered to be a promising technology to tackle the global interconnect scaling problem for terascale integrated circuits (ICs). Three-dimensional ICs typically employ through-silicon-vias (TSVs) to vertically connect planar circuits. Due to its immature fabrication process, several defects, such as void, misalignment, and dust contamination, may be introduced. These defects can significantly increase current densities within TSVs and cause severe electromigration (EM) effects, which can degrade the reliability of 3-D ICs considerably. In this paper, we propose an effective framework to mitigate EM effect of the defective TSV. At first, we analyze various possible TSV defects and their impacts on EM reliability. Based on the observation that EM can be significantly alleviated by self-healing effect, we design an EM mitigation module to protect defective TSVs from EM. To guarantee EM mitigation efficiency, we propose two defective TSV protection schemes, i.e., neighbor sharing and global sharing. Experimental results show that the global-sharing scheme performs the best and can improve the EM mean time to failure by more than $70\times $ on average with only 0.7% area overhead and less than 0.5% performance degradation compared with naked design without any EM protection.