JEP 353: Reimplement the Legacy Socket API

OwnerAlan Bateman
StatusClosed / Delivered
Componentcore-libs /
Discussionnet dash dev at openjdk dot java dot net
Reviewed byBrian Goetz, Chris Hegarty, Michael McMahon
Endorsed byBrian Goetz
Created2019/02/06 13:49
Updated2020/09/14 16:11


Replace the underlying implementation used by the and APIs with a simpler and more modern implementation that is easy to maintain and debug. The new implementation will be easy to adapt to work with user-mode threads, a.k.a. fibers, currently being explored in Project Loom.


The and APIs, and their underlying implementations, date back to JDK 1.0. The implementation is a mix of legacy Java and C code that is painful to maintain and debug. The implementation uses the thread stack as the I/O buffer, an approach that has required increasing the default thread stack size on several occasions. The implementation uses a native data structure to support asynchronous close, a source of subtle reliability and porting issues over the years. The implementation also has several concurrency issues that require an overhaul to address properly. In the context of a future world of fibers that park instead of blocking threads in native methods, the current implementation is not fit for purpose.


The and APIs delegate all socket operations to a, a Service Provider Interface (SPI) mechanism that has existed since JDK 1.0. The built-in implementation is termed the “plain” implementation, implemented by the non-public PlainSocketImpl with supporting classes SocketInputStream and SocketOutputStream. PlainSocketImpl is extended by two other JDK-internal implementations that support connections through SOCKS and HTTP proxy servers. By default, a Socket and ServerSocket is created (sometimes lazily) with a SOCKS based SocketImpl. In the case of ServerSocket, the use of the SOCKS implementation is an oddity that dates back to experimental (and since removed) support for proxying server connections in JDK 1.4.

The new implementation, NioSocketImpl, is a drop-in replacement for PlainSocketImpl. It is developed to be easy to maintain and debug. It shares the same JDK-internal infrastructure as the New I/O (NIO) implementation so it doesn't need its own native code. It integrates with the existing buffer cache mechanism so that it doesn’t need to use the thread stack for I/O. It uses java.util.concurrent locks rather than synchronized methods so that it can play well with fibers in the future. In JDK 11, the NIO SocketChannel and the other SelectableChannel implementations were mostly re-implemented with the same goal in mind.

The following are a few points about the new implementation:

ServerSocket is modified to use NioSocketImpl (or PlainSocketImpl) by default. It no longer uses the SOCKS implementation.

The SocketImpl implementations to support SOCKS and HTTP proxy servers are modified to delegate so they can work with the old and new implementations.

The instrumentation support for socket I/O in Java Flight Recorder is modified to be independent of the SocketImpl so that socket I/O events can be recorded when running with either the new, old, or custom implementations.

To reduce the risk of switching the implementation after more than twenty years, the old implementation will not be removed. The old implementation will remain in the JDK and a system property will be introduced to configure the JDK to use the old implementation. The JDK-specific system property to switch to the old implementation is If set, or set to the value true, at startup, then the old implementation will be used. Some future release will remove PlainSocketImpl and the system property.

This JEP does not propose to provide an alternative implementation of DatagramSocketImpl at this time (DatagramSocketImpl is the underlying implementation that instances of delegate to). The built-in default implementation (PlainDatagramSocketImpl) is a maintenance (and porting) burden and may be the subject of another JEP.


The existing tests in the jdk/jdk repository will be used to test the new implementation. The jdk_net test group has accumulated many tests for networking corner case scenarios over the years. Some of the tests in this test group will be modified to run twice, the second time with to ensure that the old implementation does not bit-rot during the time that the JDK includes both implementations.

A lot of code today makes direct or indirect use of libraries that use APIs defined in java.nio.channels rather than the and APIs. Every effort will be made to create awareness of the proposal and encourage developers that have code using Socket and ServerSocket to test their code with the early-access builds that are published on or elsewhere.

The microbenchmarks in the jdk/jdk repository include benchmarks for socket read/write and streaming. These benchmarks have been improved to make it easy to compare the old and new implementations. As things stand, the new implementation is about the same or 1-3% better than the old implementation on the socket read/write tests.

Risks and Assumptions

The primary risk of this proposal is that there is existing code that depends on unspecified behavior in corner cases where the old and new implementations behave differently. The differences that have been identified so far are listed here; all but the first two can be mitigated by running with

Aside from behavioral differences, the performance of the new implementation may differ to the old when running certain workloads. In the old implementation several threads calling the accept method on a ServerSocket will queue in the kernel. In the new implementation, one thread will block in the accept system call, the others will queue waiting to acquire a java.util.concurrent lock. Performance characteristics may differ in other scenarios too.

Finally, there may be instrumentation agents or tools that instrument the non-public and classes to get I/O events. These classes are not used by the new implementation.