随着市场经济的发展和人们法治意识的提高,市场主体往往通过订立合同的方式来确定合同当事人的权利和义务关系。由于主观、客观因素的限制,有些市场主体订立的合同被法院确认无效,导致合同当事人的预期目的落空甚至造成巨大的经济损失。那么,在司法实践中,法官如何认定合同无效?合同无效又会产生哪些法律后果呢?在签订合同时,如何避免签订无效合同呢? With the development of market economy and people's awareness of the rule of law, market players often determine the rights and obligations of contract parties by entering into contracts. Due to subjective and objective factors, contracts concluded by some market entities are invalidated by the court, resulting in the loss of the intended purpose of the parties to the contract and even causing huge economic losses. So, in judicial practice, how does the judge find the contract invalid? What legal consequences will the contract invalid? In signing the contract, how to avoid signing an invalid contract?