stm32无法进入串口中断

int main()
{
GPIO_InitTypeDef GPIO_InitStruct;
USART_InitTypeDef USART_InitStruct;
NVIC_InitTypeDef NVIC_InitStruct;

    RCC_AHB1PeriphClockCmd(RCC_AHB1Periph_GPIOA,ENABLE);
    RCC_APB2PeriphClockCmd(RCC_APB2Periph_USART1,ENABLE);    

    GPIO_InitStruct.GPIO_Mode = GPIO_Mode_AF;
    GPIO_InitStruct.GPIO_OType = GPIO_OType_PP;
    GPIO_InitStruct.GPIO_Pin = GPIO_Pin_9 | GPIO_Pin_10;
    GPIO_InitStruct.GPIO_PuPd = GPIO_PuPd_NOPULL;
    GPIO_InitStruct.GPIO_Speed = GPIO_Speed_100MHz;

    GPIO_Init(GPIOA,&GPIO_InitStruct);
    GPIO_PinAFConfig(GPIOA,GPIO_PinSource9,GPIO_AF_USART1);
    GPIO_PinAFConfig(GPIOA,GPIO_PinSource10,GPIO_AF_USART1);

    USART_InitStruct.USART_BaudRate = 9600;
    USART_InitStruct.USART_HardwareFlowControl = USART_HardwareFlowControl_None;
    USART_InitStruct.USART_Mode = USART_Mode_Rx | USART_Mode_Tx;
    USART_InitStruct.USART_Parity = USART_Parity_No;
    USART_InitStruct.USART_StopBits = USART_StopBits_1;
    USART_InitStruct.USART_WordLength = USART_WordLength_8b;
    
    USART_Init(USART1,&USART_InitStruct);    
    USART_ITConfig(USART1,USART_IT_RXNE,ENABLE);
    USART_Cmd(USART1,ENABLE);
    
    NVIC_PriorityGroupConfig(NVIC_PriorityGroup_2);
    NVIC_InitStruct.NVIC_IRQChannel = USART1_IRQn;
    NVIC_InitStruct.NVIC_IRQChannelCmd = ENABLE;
    NVIC_InitStruct.NVIC_IRQChannelPreemptionPriority = 0x02;
    NVIC_InitStruct.NVIC_IRQChannelSubPriority = 0x02;
    
    NVIC_Init(&NVIC_InitStruct);

while(1)
{
    USART_SendData(USART1,1);
    delay_ms(1000);
}

}

void USART1_IRQHandler(void)
{
USART_SendData(USART1,4);
if(USART_GetITStatus(USART1,USART_IT_RXNE) == SET)
{
USART_SendData(USART1,3);
}
USART_ClearITPendingBit(USART1,USART_IT_RXNE);
}