Java arena types explained

Introduction

When working with native memory in Java using the Foreign Function & Memory API, you'll need to use Arenas. An Arena controls the lifecycle of native memory segments, making sure memory is allocated and deallocated properly. In this post, we'll look at the different types of Arenas available, when to use them, and how to create your own Arena subclass.

Types of Arenas

Java provides four different types of Arenas, each with its own characteristics and use cases:

Global Arena

The global arena is the simplest type. It has these characteristics:

• Unbounded lifetime (memory is never deallocated)
• Can be accessed from any thread
• Cannot be explicitly closed

Here's an example of how to create and use a global arena:

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// Create a segment using the global arena
MemorySegment segment = Arena.global().allocate(100, 1);

// Use the segment
// Note: This segment will never be deallocated

Automatic Arena

The automatic arena lets the garbage collector manage the memory lifecycle.

It has these characteristics:

• Bounded lifetime managed by the garbage collector
• Can be accessed from any thread
• Cannot be explicitly closed

Example usage:

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// Create a segment using an automatic arena
MemorySegment segment = Arena.ofAuto().allocate(100, 1);

// When segment becomes unreachable, memory can be reclaimed
segment = null; // Memory becomes eligible for garbage collection

Confined Arena

The confined arena gives you explicit control over memory deallocation with thread safety guarantees.

It has these characteristics:

• Bounded lifetime that you control
• Can only be accessed by the thread that created it
• Must be explicitly closed

The following example shows you how to use a confined arena:

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try (Arena arena = Arena.ofConfined()) {
    MemorySegment segment = arena.allocate(100, 1);
    // Use the segment
    
} // Memory is deallocated here when arena is closed

Shared Arena

The shared arena combines manual control with multi-thread access.

It has these characteristics:

• Bounded lifetime that you control
• Can be accessed by multiple threads
• Must be explicitly closed
• Any thread can close it

Example usage:

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try (Arena arena = Arena.ofShared()) {
    MemorySegment segment = arena.allocate(100, 1);
    // Multiple threads can access this segment safely
    
} // Memory is deallocated here when arena is closed

When to Use Each Arena Type

Choosing the right arena depends on your specific needs:

1. Global Arena: Use when you need memory that lives for the entire duration of your application.

2. Automatic Arena: Great for when you want simple memory management and don't need explicit control over when memory is freed. Let the garbage collector handle it.

3. Confined Arena: Perfect for single-threaded applications where you want explicit control over memory deallocation. This is often the best choice for most use cases as it provides good performance with strong safety guarantees.

4. Shared Arena: Use when you need to share memory segments between multiple threads and want explicit control over deallocation.

Arena Characteristics Summary

Here's a quick reference table showing the key characteristics of each arena type:

Best Practices

When working with arenas, keep these tips in mind:

1. Always use try-with-resources for confined and shared arenas:

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try (var arena = Arena.ofConfined()) {
    // Work with the arena
} // Automatic cleanup

2. Choose the most restrictive arena that meets your needs:

• Start with confined arenas for single-thread scenarios
• Only use shared arenas when multiple threads truly need access
• Consider automatic arenas for simpler memory management
• Use global arenas sparingly and only when necessary

3. Be aware of thread ownership:

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var arena = Arena.ofConfined();
// This will throw WrongThreadException if called from another thread
otherThread.execute(() -> arena.allocate(100));

Creating Your Custom Arena

Sometimes the built-in arenas don't quite fit your needs. Let's create a custom arena that uses a ring buffer allocation strategy. This example shows how to implement your own arena with specific allocation behavior:

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public class RingBufferArena implements Arena {
    private final Arena baseArena;
    private final MemorySegment buffer;
    private long currentPosition = 0;
    private final long size;
    
    public RingBufferArena(long size) {
        this.size = size;
        this.baseArena = Arena.ofConfined();
        this.buffer = baseArena.allocate(size);
    }
    
    @Override
    public MemorySegment allocate(long byteSize, long byteAlignment) {
        // Reset position if we've reached the end
        if (currentPosition + byteSize > size) {
            currentPosition = 0;
        }
        
        // Create a segment from our buffer
        MemorySegment segment = buffer.asSlice(currentPosition, byteSize);
        currentPosition += byteSize;
        
        return segment;
    }
    
    @Override
    public void close() {
        baseArena.close();
    }
    
    @Override
    public MemorySegment.Scope scope() {
        return baseArena.scope();
    }
}

This custom arena implements a simple ring buffer strategy where allocations wrap around when they reach the end of the buffer. You can use it like this:

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try (RingBufferArena ringArena = new RingBufferArena(1000)) {
    MemorySegment segment1 = ringArena.allocate(100, 1);
    MemorySegment segment2 = ringArena.allocate(200, 1);
    // Use the segments
    
} // All memory is freed when the arena closes

Conclusion

In this post, we looked at the different types of Arenas available in Java's Foreign Function & Memory API. We covered when to use each type and how to create your own custom Arena implementation.