{"id":739,"date":"2018-01-26T11:13:31","date_gmt":"2018-01-26T09:13:31","guid":{"rendered":"http:\/\/www.loicmathieu.fr\/wordpress\/?p=739"},"modified":"2018-01-26T11:13:31","modified_gmt":"2018-01-26T09:13:31","slug":"les-optimisations-de-performances-de-java-9","status":"publish","type":"post","link":"https:\/\/www.loicmathieu.fr\/wordpress\/informatique\/les-optimisations-de-performances-de-java-9\/","title":{"rendered":"Java 9 performance optimizations"},"content":{"rendered":"

In a previous article on Java 9, I listed all the main new features for the developers : http:\/\/www.loicmathieu.fr\/wordpress\/en\/informatique\/les-nouveautes-de-java-9-pour-les-developeurs<\/a><\/a>.<\/p>\n

Here, I will list all the main performance optimizations of Java 9.<\/p>\n

I will again go through the main JEP :<\/p>\n

JEP 143: Improve Contended Locking<\/h2>\n

Optimization of Java monitors (lock optimization) when contended (when multiple threads access the same monitor). These are changes inside the JVM that allow siginficant (x2) performance boost on some JVM operations on lock primitives on objects (synchronize, wait, …).<\/p>\n

More information in the following presentation by Monica Beckwith :<\/p>\n

https:\/\/www.slideshare.net\/MonicaBeckwith\/jfokus-java-9-contended-locking-performance<\/a><\/a><\/p>\n

JEP 193: Variable Handles<\/h2>\n

A new API in Java, targeting advanced users, allowing to replace the usage of Unsafe<\/strong> for atomically accessing in a performant way variables.<\/p>\n

A variable handle is a typed reference to a variable, which supports read and write access to the variable under a variety of access modes. Supported variable kinds include instance fields, static fields and array elements.<\/blockquote>\n

More information in the description of the JEP : http:\/\/openjdk.java.net\/jeps\/193<\/a><\/a><\/p>\n

JEP 197: Segmented Code Cache<\/h2>\n

The code cache (a part of the Metaspace) has been separated by code types (method, non-method, profiled, non-profiled) for performance optimization and allowing future optimizations.<\/p>\n

More information in the description of the JEP : http:\/\/openjdk.java.net\/jeps\/197<\/a><\/a><\/p>\n

JEP 254: Compact Strings<\/h2>\n

That’s one of the main JEP around performance!<\/p>\n

The java String implementation was reworked to allow a more compact version.<\/p>\n

Until now, Strings was stored in UTF-16, each character took 2 bytes as stored in an array of char.<\/p>\n

With Compact String, at the String construction time, if the String is ISO-8859-1 (or Latin-1) compatible, each character will then be stored in only one byte, the String storage evolves from an array of char to an array of bytes. The potential gain is huge : almost 50% shrinking of the size needed to store Strings. Strings are usually the main source of memory usage and generally they are ISO-8859-1 compatible.<\/p>\n

If the String is not ISO-8859-1 compatible, then characters are still stored in a bytes array, but you will need two bytes for each one. A flag was added to the String class to know the encoding of this one.<\/p>\n

This change has been made without any changes on the public API of Java : by using Java 9, you directly use Compact Strings! StringBuilder\/Buffer was also updated to take advantage of it.<\/p>\n

During preliminary tests for the implementation of this JEP, the following gains have been seen (see http:\/\/cr.openjdk.java.net\/~shade\/density\/state-of-string-density-v1.tx<\/a><\/a>t) :<\/p>\n