The Java compiler must know,while it is creating the program,the exact size and lifetime of all the data that is stored on the stack,because it must generate the code to move the stack pointer up and down.

• 创建程序时，Java编译器必须准确地知道堆栈内保存的所有数据的“长度”以及“存在时间”。这是由于它必须生成相应的代码，以便向上和向下移动指针。