The new I/O (NIO) classes in java.nio allow the creation and use of direct buffers. These buffers tremendously increase throughput for repeated I/O activities. However, their creation and reclamation is more expensive than the creation and reclamation of heap-based nondirect buffers because direct buffers are managed using OS-specific native code. This added management cost makes direct buffers a poor choice for single-use or infrequently used cases. Direct buffers are also outside the scope of Java's garbage collector; consequently, injudicious use of direct buffers can cause memory leaks. Finally, frequent allocation of large direct buffers can cause an OutOfMemoryError.

Noncompliant Code Example

This noncompliant code example uses both a short-lived local object, rarelyUsedBuffer, and a long-lived, heavily used object, heavilyUsedBuffer. Both are allocated in nonheap memory; neither is garbage collected.

ByteBuffer rarelyUsedBuffer = ByteBuffer.allocateDirect(8192);
// Use rarelyUsedBuffer once

ByteBuffer heavilyUsedBuffer = ByteBuffer.allocateDirect(8192);
// Use heavilyUsedBuffer many times

Compliant Solution

This compliant solution uses an indirect buffer to allocate the short-lived, infrequently used object. The heavily used buffer appropriately continues to use a nonheap, non-garbage-collected direct buffer.

ByteBuffer rarelyUsedBuffer = ByteBuffer.allocate(8192);
// Use rarelyUsedBuffer once

ByteBuffer heavilyUsedBuffer = ByteBuffer.allocateDirect(8192);
// Use heavilyUsedBuffer many times

Applicability

Direct buffers are beyond the scope of Java's garbage collector and can cause memory leaks if they are used injudiciously. In general, direct buffers should be allocated only when their use provides a significant gain in performance.

Bibliography

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