What are the different types of garbage collection in Java?
What are the different types of garbage collection in Java?
I'll respond in English for this question.
In Java, there are two primary methods of garbage collection: mark-and-sweep and concurrent low-pause garbage collection.
Mark-and-Sweep
This is the default garbage collector used by most JVMs (Java Virtual Machines). The process works as follows:
Mark phase: The garbage collector identifies all objects in the heap that are still reachable from the Java stack or other garbage-collected data structures. This includes any static variables, method invocation stacks, and data structures created during program execution. Sweep phase: Once the mark phase is complete, the garbage collector iterates through the marked objects and frees memory space occupied by unmarked (garbage) objects.The mark-and-sweep algorithm has several limitations:
It can cause long pause times, especially if the heap contains many large objects or a large number of small objects. It may not be suitable for real-time systems where low-latency garbage collection is essential.Concurrent Low-Pause Garbage Collection
This type of garbage collection was introduced in Java 6 to address the limitations of mark-and-sweep. The concurrent collector runs simultaneously with the application, allowing it to collect garbage without disrupting the normal execution of the program:
Background thread: A separate thread (the garbage collector) continuously monitors heap objects and identifies garbage as objects become unreachable. Low-pause intervals: The garbage collector periodically collects garbage in short, low-pause intervals (typically 10-50 ms), allowing the application to continue running without interruption.The concurrent low-pause garbage collector is more suitable for systems that require a balance between performance and latency. However:
It can lead to higher CPU usage due to the added overhead of running a separate thread. The pause times may still be significant if the heap contains many large objects or a large number of small objects.Other Garbage Collection Algorithms
In addition to mark-and-sweep and concurrent low-pause garbage collection, there are other algorithms used in Java, including:
Generational Garbage Collection: This algorithm divides the heap into generations based on object age (newly created vs. long-lived) and uses different garbage collection strategies for each generation. Incremental Garbage Collection: The garbage collector collects garbage incrementally by periodically interrupting the application to collect garbage, rather than using a single, longer pause time. Asynchronous Garbage Collection: Similar to concurrent low-pause garbage collection, this algorithm uses an asynchronous thread to collect garbage without disrupting the application.Each garbage collection algorithm has its strengths and limitations, making it essential for developers to consider their specific needs and performance requirements when choosing the best garbage collection strategy for their Java applications.
What is the default garbage collector in Java 17
The default garbage collector in Java 17 is the Garbage-First (G1) Collector.
G1 has been a part of the JDK since Java 7 Update 11 and has become the default garbage collector for many platforms since then. It's an incremental, concurrent garbage collector that aims to minimize pause times while keeping the overall memory footprint reasonable.
Here are some key characteristics of G1:
Incremental: G1 runs in short periods, called "incremental garbage collection" (IGC), which reduces pauses and allows the application to continue running. Concurrent: The collector works concurrently with the application, minimizing the impact on the system. This is achieved by dedicating a thread for IGC, making sure that it's not blocking other threads. Generational: G1 divides heap objects into three generations: young, old, and humongous. It prioritizes garbage collection of younger objects first, then moves up to older ones as needed. Region-based: G1 divides the heap into fixed-size regions (typically 512 MB). The collector focuses on a specific region at a time, ensuring that it doesn't interfere with other parts of the heap. Low-pause: G1 strives for low pause times by using various techniques: Young garbage collection: Quickly collects young objects to prevent them from escaping. Incremental promotion: Promotes objects from one generation to another in small chunks, reducing the need for full-garbage collection pauses.Java 17 introduces some improvements to G1, including better handling of large heaps and more efficient incremental garbage collection. The JVM can adjust the GC behavior based on the application's characteristics, like heap size, allocation patterns, and available CPU resources.
In addition to G1, Java 17 provides alternative garbage collectors:
ZGC (Z Garbage Collector): A low-pause, concurrent, generational collector that uses a mark-and-sweep algorithm. Shenandoah: A low-pause, concurrent, generational collector that uses an incremental approach for young and old objects.When deciding which garbage collector to use in your Java 17 application, consider factors like:
Heap size: G1 might be a better choice for larger heaps due to its ability to handle them efficiently. Pause-time constraints: If you need extremely low pause times, G1 or ZGC might be suitable for most applications. Allocation patterns: Applications with frequent short-lived object creation might benefit from Shenandoah's incremental approach.Remember that the best garbage collector depends on your specific use case and requirements. Always monitor and adjust GC settings according to your application's behavior to ensure optimal performance.
