Research: Different Types of JVM Garbage Collection Mechanisms

In-depth analysis: Several types of JVM garbage collection mechanisms, specific code examples are required

[Introduction]
JVM (Java Virtual Machine) is a Java program Runtime environment, which is responsible for compiling Java code into bytecode and executing it. When a Java application is running, the JVM automatically manages memory resources, the most important of which is garbage collection. Garbage collection refers to cleaning up useless objects and releasing occupied memory space, thereby improving resource utilization and application performance. This article will provide an in-depth analysis of several types of JVM garbage collection mechanisms and provide specific code examples.

[Mark-Sweep]
Mark-Sweep is one of the most basic garbage collection algorithms. Its main steps include marking and clearing phases.

Marking phase: Starting from the root object, traverse the entire object graph, and mark all objects reachable from the root object as surviving objects.
The sample code is as follows:

public void mark(Object obj) { if (obj.isMarked) { return; } obj.isMarked = true; for (Object ref : obj.references) { mark(ref); } } public void markSweep() { mark(rootObject); for (Object obj : heap) { if (!obj.isMarked) { sweep(obj); } else { obj.isMarked = false; } } } public void sweep(Object obj) { obj.references.clear(); heap.remove(obj); }

[Copying]
The copying algorithm is another common garbage collection algorithm. It divides the heap memory into two areas and only uses one of them at a time. When you are done with one area, copy the surviving objects to another area, and then clear all objects in the original area.

The sample code is as follows:

public void copy() { for (Object obj : heap) { if (obj.isMarked) { copyToSurvivorSpace(obj); } } } public void copyToSurvivorSpace(Object obj) { if (obj.isCopied) { return; } obj.isCopied = true; SurvivorSpace.add(obj); for (Object ref : obj.references) { copyToSurvivorSpace(ref); } } public void swap() { Object[] temp = fromSpace; fromSpace = toSpace; toSpace = temp; } public void clear() { toSpace.clear(); }

[Mark-Compact]
Mark-Compact algorithm is also one of the common garbage collection algorithms. Its main steps include three stages: marking, sorting and clearing.

Marking phase: Same as mark-clear algorithm, mark all objects reachable from the root object as alive objects.
Decluttering phase: Move all surviving objects to one end, clean up useless objects, and update reference relationships.
Clear phase: Delete the memory space occupied by useless objects.

The sample code is as follows:

public void compact() { int newIndex = 0; for (int i = 0; i < heap.length; i++) { Object obj = heap[i]; if (obj.isMarked) { obj.isMarked = false; heap[newIndex++] = obj; } else { sweep(obj); } } } public void swap() { for (Object obj : heap) { for (Field field : obj.fields) { if (field.getValue().isObject()) { field.getValue().updateReference(); } } } } public void sweep(Object obj) { obj.references.clear(); heap.remove(obj); }

[Summary]
This article provides an in-depth analysis of several types of JVM garbage collection mechanisms and provides specific code examples. The mark-sweep algorithm frees up memory space by marking live objects and clearing unused objects. The copy algorithm collects garbage by copying live objects to another memory area. The mark-organize algorithm reclaims memory space by marking surviving objects, organizing object locations, and clearing useless objects. Different algorithms differ in implementation details and applicable scenarios. Developers can choose an appropriate garbage collection algorithm based on specific circumstances to improve application performance.

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