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Sockets Java API Overview

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Note: Sockets are only available for paid apps, and traffic from sockets is billed as outgoing bandwidth. Sockets are also limited by daily and per minute (burst) quotas. App Engine supports regular outbound Java sockets such as java.net.Socket and java.net.DatagramSocket. There is currently no support for sockets via java.nio.SocketChannel or other java.nio classes. The Sockets API allows client code to call get/set options against sockets. (Previously, calls raised Not Implemented exceptions.)

The currently supported options are:

  • SO_KEEPALIVE
  • SO_DEBUG
  • TCP_NODELAY
  • SO_LINGER
  • SO_OOBINLINE
  • SO_SNDBUF
  • SO_RCVBUF
  • SO_REUSEADDR

Limitations and restrictions

Although App Engine supports sockets, there are certain limitations and behaviors you need to be aware of when using sockets:

  • Sockets are available only for paid apps.
  • You cannot create a listen socket; you can only create outbound sockets.
  • FTP is not supported.
  • java.net.URL is still configured to use the URL Fetch API; there is currently no way around this.
  • Most classes in javax.net.ssl are supported.
  • You can only use TCP or UDP; arbitrary protocols are not allowed.
  • You cannot bind to specific IP addresses or ports.
  • Port 25 (SMTP) is blocked; you can still use authenticated SMTP on the submission port 587.
  • Private, broadcast, multicast, and Google IP ranges (except those whitelisted below), are blocked:

    • Google Public DNS: 8.8.8.8, 8.8.4.4, 2001:4860:4860::8888, 2001:4860:4860::8844 port 53
    • Gmail SMTPS: smtp.gmail.com port 465 and 587
    • Gmail POP3S: pop.gmail.com port 995
    • Gmail IMAPS: imap.gmail.com port 993
  • Socket descriptors are associated with the App Engine app that created them and are non-transferable (cannot be used by other apps).

  • Sockets may be reclaimed after 2 minutes of inactivity; any socket operation keeps the socket alive for a further 2 minutes.
  • You cannot Select between multiple available sockets because that requires java.nio.SocketChannel which is not currently supported.)

Using sockets with the development server

You can run and test code using sockets on the development server, without using any special command line parameters.

App Engine sample using sockets

For a sample using sockets, see the socket demo app in the Google Cloud Platform GitHub.