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Community Blog Understanding Garbage Collection in Java

Understanding Garbage Collection in Java

In this article, we will explain the Garbage collection (GC) mechanism in Java via the explanation on GC and how GC works.

Garbage collection (GC), as its name implies, is a means of freeing space occupied by waste materials, or garbage, and avoid memory leaks. Through performing the GC mechanism, available memory can be effectively used. Moreover, through this process, objects that are dead or unused for a long time in the memory heap will be deleted and the memory space used by these objects will be reclaimed.

GC on the Java virtual machine (JVM) follows the Stop-The-World mechanism. Stop-The-World means that the execution of the program is suspended for GC till all objects in the heap are processed.

In Java, GC roots can be four types of objects:

  1. Objects referenced in the virtual machine (VM) stack, that is the local variable table in the stack frame
  2. Objects referenced by class static attributes in the method area
  3. Objects referenced by constants in the method area
  4. Objects referenced by JNI (the Native method) in the native method stack

After determining the garbage to be collected, the garbage collector starts its work. However, this involves a question: How can we efficiently perform GC? JVM specifications do not clearly define how to implement the garbage collector. Therefore, VMs from different manufacturers can implement the garbage collector in different ways.

  1. Mark-Sweep Algorithm: It is the most common garbage collection algorithm, which performs two operations. It first marks the objects to be garbage-collected in the memory space and then clears the marked objects up from the heap. The algorithm has a clear logic and is easy for operation. However, it has a big problem of memory fragmentation.
  2. Copying Algorithm: It evolves from the Mark-Sweep algorithm to solve the problem of memory fragmentation. This algorithm divides available memory into two equally sized semi-spaces. Only one semi-space is active at a time.
  3. Mark-Compact Algorithm: It has the same marking process as the Mark-Sweep algorithm. However, this algorithm does not directly clear up the objects that can be garbage-collected. Instead, it moves all living objects to one end, and then reclaims the memory space beyond the end boundary.
  4. Generational Collection Algorithm: It is not an idea or a theory, but a combination of the first three algorithms. It provides the combination of different algorithms for different scenarios.

For more detailed information, please go to How Does Garbage Collection Work in Java?.

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