Java 15 面试题与答案

问题 6. What Is Biased Locking in Java 15?

Biased locking is an optimization of thread synchronization aimed at reducing synchronization overhead when the same monitor is repeatedly acquired by the same thread (i.e., when the same thread repeatedly calls code synchronized on the same object).

In the example above, this means that the first time the add() method is called, the vector monitor is biased to the thread in which the test method is executed. This bias speeds up the monitor's acquisition in the following 9,999,999 add() method calls.

问题 7. Why Was Biased Locking Disabled in Java 15?

Biased locking mainly benefits legacy applications that use data structures such as Vector, Hashtable, or StringBuffer, where each access is synchronized.

Modern applications usually use non-synchronized data structures such as ArrayList, HashMap, or StringBuilder – and the data structures optimized for multithreading in the java.util.concurrent package.

Because the code for biased locking is highly complex and deeply intertwined with the JVM code, it requires a great deal of maintenance and makes changes within the JVM's synchronization system costly and error-prone.

Therefore, the JDK developers decided in JDK Enhancement Proposal 374 to disable biased locking by default, mark it as "deprecated" in Java 15 and remove it entirely in one of the following releases.

问题 8. What are Specialized Implementations of TreeMap Methods in Java 15?

In TreeMap, specialized methods putIfAbsent(), computeIfAbsent(), computeIfPresent(), compute(), and merge() were implemented.

These methods were only specified as default methods in the Map interface since Java 8.

The TreeMap-specific implementations are optimized for the underlying red-black tree; accordingly, they are more performant than the interface's default methods.

问题 9. What are the Deprecations and Deletions in Java 15?

• Remove the Nashorn JavaScript Engine
• Remove the Solaris and SPARC Ports
• Deprecate RMI Activation for Removal

问题 10. Provide the release notes of Java 15.

The details are provided here.