What’s new in Java 27 for developers?

The HashMap.putAll() optimization is a nice example of a change that can have a direct performance benefit. When the source map is also a HashMap, the implementation can use a faster specialized path instead of handling every entry through the general mechanism.

G1 remains a practical general-purpose choice because it offers a reasonable balance between memory usage, CPU overhead, and pause times. ZGC and Shenandoah target very low latency, but they generally consume more memory and CPU, so they make more sense for large heaps or applications where sub-millisecond pauses are especially important.

The preview features are probably the biggest attraction for many developers, although it’s still worth treating them as experimental and checking how they evolve in future releases. Java 27 also includes smaller performance improvements that may matter in everyday applications.

Compact object headers have already been used extensively in production by organizations such as Amazon and SAP, including through backports and downstream JDK distributions. That doesn’t necessarily mean the feature is enabled by default in every build of a particular JDK vendor, so it’s best to check that distribution’s release notes and runtime defaults.