Automatic software development has been a research hot spot in the field of software engineering (SE) in the past decade. In particular, deep learning (DL) has been applied and achieved a lot of progress in various SE tasks. Among all applications, automatic code generation by machines as a general concept, including code completion and code synthesis, is a common expectation in the field of SE, which may greatly reduce the development burden of the software developers and improves the efficiency and quality of the software development process to a certain extent. Code completion is an important part of modern integrated development environments (IDEs). Code completion technology effectively helps programmers complete code class names, method names, and key-words, etc., which improves the efficiency of program development and reduces spelling errors in the coding process. Such tools use static analysis on the code and provide candidates for completion arranged in alphabetical order. Code synthesis is implemented from two aspects, one based on input-output samples and the other based on functionality description. In this study, we introduce existing techniques of these two aspects and the corresponding DL techniques, and present some possible future research directions.