With the continuous development of computer hardware and software, memory is getting bigger and bigger, and garbage collection (Garbage Collection, GC) has also become a very important part of modern programming languages. In the Java language, the garbage collector is a part of the Java Virtual Machine (JVM), which is responsible for managing the memory usage and release of the Java program to ensure the normal operation of the program. This article will introduce the garbage collection algorithm in the JVM in detail from the perspectives of basic principles of garbage collection, GC algorithm classification, JVM garbage collector and optimization, etc., to help readers better understand and master the GC recycling mechanism.
1. Basic principles of garbage collection
Before starting to understand the garbage collection algorithm in depth, we first need to understand the basic principles of garbage collection. In Java, there are two types of memory used by programs: heap memory and stack memory. Among them, heap memory is used to store object instances, while stack memory is used to store data such as method calls and basic type variables.
When an object has no references, it becomes garbage and can be reclaimed by the garbage collector. The garbage collector scans the objects in the heap memory, determines which objects are garbage, and frees them for use by other parts of the program.
During garbage collection, the garbage collector usually performs the following steps:
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Mark: traverse all objects in the heap memory and mark those objects that are still referenced by live objects.
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Clear: Clear those objects that have not been marked to release the space they occupy.
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Collation: Move all remaining live objects to a port on the heap to provide a larger contiguous space for the next allocation of objects.
2. GC algorithm classification
In Java, garbage collection algorithms can be divided into two categories: garbage collection algorithms based on reference counting and garbage collection algorithms based on reachability analysis.
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Garbage collection algorithm based on reference counting: Add a reference counter to each object. When a pointer refers to the object, the counter is incremented by 1, so that when the counter is reduced to 0, it means that the object has become garbage.
However, this algorithm has a fatal problem: it cannot solve the circular reference problem. If two objects refer to each other, their reference counters will not be 0, and the garbage collector will not be able to recycle them. Therefore, in actual development, garbage collection algorithms based on reference counting are not commonly used.
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Garbage collection algorithm based on reachability analysis: starting from the GC Roots object, search all referenced objects from top to bottom. During the search, if an object does not have any references, it means that the object has become garbage.
This algorithm can solve the circular reference problem, because as long as an object can be reached from the GC Roots object, then it will be considered as a live object, even if they refer to each other.
3. JVM Garbage Collector
The JVM garbage collector is the component responsible for performing garbage collection. In Java, garbage collectors are mainly divided into the following types:
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Serial collector: A single-threaded garbage collector that uses only one thread for garbage collection. Good for small applications or lightweight servers because of its high pause times.
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Parallel collector: A multi-threaded garbage collector that uses multiple threads simultaneously during garbage collection. Suitable for large applications and high concurrent servers.
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CMS collector: A concurrent, low-pause garbage collector. It uses multiple threads to work simultaneously to complete garbage collection in the shortest possible time. Since it's concurrent, it won't affect the execution of the main thread. However, the CMS collector performs less efficiently.
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G1 collector: An efficient garbage collector for server-side applications. It performs garbage collection in units of the entire heap, and divides the heap into multiple regions to facilitate batch and parallel processing of garbage collection. When performing G1 garbage collection, each partition can be prioritized so that garbage collection completes in the shortest possible time.
4. GC optimization
During the development process, we need to minimize the number of GC executions and the pause time of garbage collection to improve the performance and reliability of the program. Some GC optimization methods are listed below:
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Minimize the number of objects created.
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Try to use primitive data types instead of object types.
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Reduce the life cycle of the object.
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Manually call the System.gc() method to notify the virtual machine to perform garbage collection.
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Use an appropriate heap size to avoid frequent garbage collections.
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Use appropriate GC algorithms to reduce GC pause times.
5. Summary
Garbage collection is a very important part of modern programming languages, and Java's garbage collection mechanism is also one of the research hotspots. Through the introduction of this article, we understand the basic principles of garbage collection, GC algorithm classification, JVM garbage collector and optimization, etc., so as to better understand and master the GC recycling mechanism. In actual work, we need to select the appropriate GC algorithm and optimization method according to specific business needs to improve the performance and reliability of the program and meet the needs of users.