What is SSL context?

• SSLContext: Instances of this class represent a secure socket protocol implementation which acts as a factory for secure socket factories or SSLEngines. This class is initialized with an optional set of key and trust managers and source of secure random bytes.

• SSLSession: In SSL, sessions are used to describe an ongoing relationship between two entities. Each SSL connection involves one session at a time, but that session may be used on many connections between those entities, simultaneously or sequentially. The session used on a connection may also be replaced by a different session. Sessions are created, or rejoined, as part of the SSL handshaking protocol. Sessions may be invalidated due to policies affecting security or resource usage, or by an application explicitly calling invalidate. Session management policies are typically used to tune performance.

• SSLSessionContext: A SSLSessionContext represents a set of SSLSessions associated with a single entity. For example, it could be associated with a server or client who participates in many sessions concurrently. An SSLSessionContext can be configured with a session timeout.

SSL Context is a collection of ciphers, protocol versions, trusted certificates, TLS options, TLS extensions etc. Since it is very common to have multiple connections with the same settings they are put together in a context and the relevant SSL connections are then created based on this context. And to create a new connection you need only refer to the context which thus saves time and memory compared to the case you would have to re-create of all these settings.

EDIT: @EJP nicely describes this "collection" as factory. A SSL context is not the same as a SSL session even both are collections of settings. A session is what you get after the SSL handshake and covers only the cipher and protocol version both parties agreed on and also the exchanged key. Whereas the context covers all the ciphers and protocol versions and also the list of trusted certificates the local system (client or server) is willing to support when establishing a new TLS connection. Thus a SSL session describes an established SSL relation while the SSL context describes what you need to establish an SSL relation.