What are the types of garbage collectors in Java?

Java provides different types of Garbage Collectors (GC) to manage memory efficiently based on application requirements.

In simple words:

Different garbage collectors are designed for different goals such as low pause time, high throughput, scalability, and low latency.

Why Multiple Garbage Collectors Exist?

Different applications have different needs:

• Banking systems need stability
• Real-time systems need low latency
• Batch systems need high throughput
• Cloud-native apps need scalability

Garbage Collector Overview Diagram

Java Garbage Collectors

      |
      +-------> Serial GC

      |
      +-------> Parallel GC

      |
      +-------> CMS GC

      |
      +-------> G1 GC

      |
      +-------> ZGC

      |
      +-------> Shenandoah GC

Main Types of Garbage Collectors

• Serial Garbage Collector
• Parallel Garbage Collector
• CMS Garbage Collector
• G1 Garbage Collector
• Z Garbage Collector (ZGC)
• Shenandoah Garbage Collector

1. Serial Garbage Collector

Serial GC uses a single thread for garbage collection.

How It Works?

Application Stops

      |
      v

Single GC Thread Runs

      |
      v

Unused Objects Removed

      |
      v

Application Resumes

Features of Serial GC

• Single-threaded
• Simple implementation
• Low memory overhead
• Stop-the-world collector

Best For

• Small applications
• Single CPU systems
• Low-memory environments

Disadvantages

• Long pause times
• Poor scalability
• Not suitable for enterprise systems

JVM Option

-XX:+UseSerialGC

2. Parallel Garbage Collector

Parallel GC uses multiple threads for garbage collection.

Why Faster?

Multiple threads clean memory simultaneously.

Parallel GC Flow

Application Paused

      |
      v

Multiple GC Threads Execute

      |
      v

Heap Cleaned Faster

      |
      v

Application Resumes

Features of Parallel GC

• Multi-threaded
• High throughput
• Faster cleanup
• Better CPU utilization

Best For

• Multi-core servers
• Batch processing systems
• Applications prioritizing throughput

Disadvantages

• Long GC pauses possible
• Not ideal for low-latency systems

JVM Option

-XX:+UseParallelGC

3. CMS Garbage Collector

CMS stands for:

Concurrent Mark Sweep

Main Goal

Reduce application pause times.

CMS Working Flow

Application Running

      |
      v

GC Runs Concurrently

      |
      v

Shorter Pause Times

Features of CMS

• Low pause times
• Concurrent execution
• Better responsiveness

Best For

• Interactive enterprise systems
• Web applications
• Response-sensitive systems

Disadvantages

• More CPU usage
• Memory fragmentation
• Deprecated in modern Java

JVM Option

-XX:+UseConcMarkSweepGC

Important Note

CMS is deprecated and removed in newer Java versions.

4. G1 Garbage Collector

G1 stands for:

Garbage First

Main Goal

Provide balanced performance with low pause times.

G1 Internal Architecture

Heap Divided into Regions

      |
      v

GC Cleans Most Garbage-Filled Regions First

Features of G1 GC

• Low pause times
• Scalable
• Efficient heap management
• Predictable performance

Best For

• Large enterprise applications
• Spring Boot applications
• Microservices
• Cloud-native systems

Advantages

• Good balance of throughput and latency
• Handles large heaps efficiently
• Modern JVM default collector

JVM Option

-XX:+UseG1GC

5. Z Garbage Collector (ZGC)

ZGC is a modern low-latency garbage collector.

Main Goal

Ultra-low pause times even with huge heaps.

ZGC Flow

Application Running

      |
      v

GC Happens Concurrently

      |
      v

Pause Times Remain Very Small

Features of ZGC

• Very low latency
• Scales to terabytes of heap
• Concurrent processing
• Minimal pauses

Best For

• Large-scale cloud systems
• Financial systems
• Real-time enterprise applications
• AI systems

Advantages

• Extremely low pause times
• Excellent scalability
• Modern cloud-native support

Disadvantages

• Higher CPU usage
• Requires newer JVM versions

JVM Option

-XX:+UseZGC

6. Shenandoah Garbage Collector

Shenandoah is another low-pause concurrent garbage collector.

Main Goal

Reduce GC pause times regardless of heap size.

Features of Shenandoah

• Concurrent compaction
• Low pause times
• Large heap support

Best For

• Large memory applications
• Cloud-native workloads
• High-performance systems

JVM Option

-XX:+UseShenandoahGC

Comparison Table

Which GC is Default in Modern Java?

Modern JVM versions use:

G1 GC

as default collector.

Why G1 Became Default?

• Balanced performance
• Low pause times
• Better scalability
• Works well for most enterprise workloads

Garbage Collectors in Banking Systems

Banking systems typically use:

• G1 GC
• ZGC

Why?

• Need low latency
• Need stable response times
• Need high throughput

Garbage Collectors in E-Commerce Systems

E-commerce platforms commonly use:

• G1 GC
• Parallel GC

Why?

• High concurrent users
• Large object creation
• Heavy request processing

Garbage Collectors in Spring Boot

Spring Boot applications usually use:

• G1 GC

Why?

• Good default performance
• Handles REST workloads efficiently
• Suitable for microservices

Microservices and Garbage Collectors

Cloud-native microservices commonly use:

• G1 GC
• ZGC

Why?

• Container optimization
• Low-latency APIs
• Scalable workloads

GC Selection Flow

Application Requirement

      |
      +-------> Small App → Serial GC

      |
      +-------> High Throughput → Parallel GC

      |
      +-------> Low Latency → ZGC/Shenandoah

      |
      +-------> Enterprise App → G1 GC

GC Monitoring Tools

• JConsole
• VisualVM
• Java Flight Recorder
• Grafana
• Prometheus

GC Monitoring Flow

Application Running

      |
      v

GC Metrics Captured

      |
      v

Heap Usage Analyzed

      |
      v

Performance Optimized

Common Interview Mistake

Many developers think all garbage collectors behave the same way.

Actually:

• Each collector is optimized for different workloads.

Another Common Mistake

Many developers think newest GC is always best.

Actually:

• GC selection depends on application requirements.

Best Practices

• Use G1 GC for most enterprise applications
• Use ZGC for ultra-low latency systems
• Monitor GC pauses regularly
• Tune heap sizes carefully
• Test GC behavior under production load

Realtime Enterprise Example

Spring Boot Microservice

java -XX:+UseG1GC
     -Xms512m
     -Xmx2g
     app.jar

Optimized for scalable cloud-native deployment.

Related Learning Topics

• What is Garbage Collection in Java
• What is Memory Management in Java
• How JVM Works Internally
• What is finalize() Method in Java
• What is String Pool in Java
• Memory Management in Java
• What is Spring Boot
• What are Microservices

Professional Interview Answer

Java provides multiple types of Garbage Collectors such as Serial GC, Parallel GC, CMS GC, G1 GC, ZGC, and Shenandoah GC. Each garbage collector is optimized for different application requirements including throughput, low latency, scalability, and pause-time reduction. Serial GC is suitable for small applications, Parallel GC focuses on throughput, CMS minimizes pause times, G1 GC provides balanced enterprise performance, while ZGC and Shenandoah are designed for ultra-low-latency large-scale systems. Modern enterprise applications, Spring Boot services, banking systems, cloud-native microservices, and distributed architectures commonly use G1 GC or ZGC depending on workload requirements.

Frequently Asked Questions

Which GC is default in modern Java?

G1 Garbage Collector is the default GC in modern Java versions.

Which GC is best for low latency?

ZGC and Shenandoah GC are best for ultra-low latency systems.

Which GC is best for small applications?

Serial GC is suitable for small applications.

Why is Parallel GC faster?

Because it uses multiple threads for garbage collection.

Is CMS GC still used?

CMS GC is deprecated and removed in newer Java versions.