Last Updated on August 18, 2023 by Mayank Dham
Garbage collection is a process in Java that automatically releases memory used by objects that the application no longer uses. It is an essential component of the Java programming language that facilitates memory management. Java memory management is based on the heap, which is a region of memory where objects and classes are stored. The heap is managed by the Java Virtual Machine (JVM), which runs the garbage collector on a regular basis to reclaim memory occupied by objects that are no longer accessible to the program. When an object can no longer be reached by any live threads or referenced by any other objects, it is considered garbage collection eligible.
What is Garbage Collection in Java?
It is a mechanism in Java that automatically frees up memory that is no longer being used by the program. The purpose of garbage collection is to simplify memory management for Java developers and reduce the risk of memory leaks.
An object is considered eligible for garbage collection when it can no longer be reached by any live threads or referenced by any other objects. The Java Virtual Machine (JVM) periodically runs the garbage collector to identify and reclaim these unused objects and the memory they occupy.
The JVM uses different algorithms to determine which objects are eligible for garbage collection. Some common algorithms include the Mark and Sweep algorithm, the Copying algorithm, and the Generational algorithm. The specific algorithm used by the JVM depends on the implementation and configuration.
It’s important to note that garbage collection in Java is non-deterministic, meaning that the timing of garbage collection and the objects that are eligible for collection cannot be predicted. Developers should not rely on garbage collection to release resources immediately or to control the timing of memory cleanup in their programs.
Types of Activities in Java Garbage Collection
There are several activities involved in the Java garbage collection process. These activities are:
- Object allocation: Objects are allocated on the heap and their memory is managed by the JVM.
- Object Reachability: The JVM determines which objects are still reachable by live threads and which objects are eligible for garbage collection. Objects that are no longer reachable are considered eligible for garbage collection.
- Object Marking: The JVM marks the eligible objects for garbage collection. This marking is used to determine which objects can be safely reclaimed.
- Object Sweeping: The JVM scans the heap and frees the memory occupied by the marked objects. This process is known as sweeping.
- Object Compaction: After sweeping, the JVM may compact the heap by moving objects to reduce fragmentation. This helps to reclaim any unused memory and improve performance.
- Object Finalization: Some objects may require special processing before they can be collected. The JVM runs the finalization process for these objects to clean up any resources they may be holding.
- Reference Queue Processing: Java provides a mechanism for managing weak references, which are references that can be cleared by the garbage collector. The JVM processes the reference queue to determine if any weak references have been cleared and release the associated objects.
These activities are performed periodically by the JVM to ensure efficient and effective memory management in Java. The specific details of the garbage collection process, including the algorithms used and the timing of garbage collection, depending on the specific implementation and configuration of the JVM.
How Does Garbage Collection in Java Works?
Garbage collection in Java works by managing the memory used by objects in a program automatically. The Java Virtual Machine (JVM) is in charge of managing the memory heap, which is a region of memory where objects and classes are stored.
The garbage collector is run by the JVM on a regular basis to reclaim memory occupied by objects that are no longer in use by the programme. The garbage collector identifies objects that are no longer reachable by any live threads or referenced by any other objects to determine which are eligible for collection. Once the eligible objects are identified, the garbage collector marks them for collection and then frees the memory they occupy. This process is known as sweeping. After sweeping, the JVM may also compact the heap to reduce fragmentation and improve performance.
The garbage collector can use different algorithms to determine which objects are eligible for collection, such as the Mark and Sweep algorithm, the Copying algorithm, and the Generational algorithm. The specific algorithm used by the JVM depends on the implementation and configuration.
In addition to the automatic memory management provided by garbage collection, Java also provides methods for manual memory management, such as the System.gc() and Runtime.gc() methods, which allow developers to request garbage collection. However, these methods only provide a hint to the JVM and do not guarantee that garbage collection will occur. Relying on these methods to control the timing of garbage collection can negatively impact performance.
Important Concepts related to Garbage Collection in Java
The Garbage Collection (GC) mechanism stands as a sentinel, responsible for identifying and reclaiming memory occupied by objects that are no longer in use. However, the nuances of GC go beyond mere memory reclamation; they extend to understanding concepts like unreachable objects, eligibility for garbage collection, and even requesting the JVM to initiate the process.
Unreachable Objects
An object is considered unreachable when it lacks any references pointing to it. This definition extends to objects marooned in an "island of isolation." For instance:
Integer i = new Integer (4);
i = null;In this scenario, the integer object ‘4’ becomes a candidate for garbage collection, making space for efficient memory usage.
Eligibility for Garbage Collection
The crux of GC lies in identifying objects eligible for disposal. An object achieves eligibility for garbage collection when it becomes unreachable. Upon setting ‘i’ to null, the integer object ‘4’ in the heap memory transitions into a state where it can be collected by the garbage collector.
Strategies for Making Objects Eligible for GC
While programmers are not directly responsible for object destruction, it’s advisable to render objects unreachable when they’re no longer needed. This can be accomplished through various means:
1. Nullifying the reference variable: By assigning null to a reference, you sever the link between the object and the variable, making it eligible for GC.
2. Re-assigning the reference variable: Redirecting the reference to a different object renders the original object unreachable.
3. Objects created inside a method: Objects created within a method scope become unreachable once the method finishes executing.
4. Island of Isolation: When a group of objects references each other but is isolated from the rest of the application, they collectively become unreachable.
Initiating Garbage Collection
While the JVM runs the Garbage Collector periodically, developers can also request its execution. Two methods facilitate this:
1. System.gc(): Invoking this method prompts the JVM to run the Garbage Collector, though there’s no guarantee of immediate execution.
2. Runtime.getRuntime().gc(): This method provides a way to interface with the JVM and request GC. Similar to System.gc(), there’s no strict assurance of immediate action.
Leverage of Finalization
Just before the ultimate destruction of an object, the Garbage Collector invokes the finalize() method on that object. It’s an opportunity to execute cleanup activities, such as closing database connections. Overriding the finalize() method from the Object class enables developers to tailor it to their needs.
Advantages of Garbage Collection in Java
Garbage collection in java is an important feature of the Java programming language and has several advantages, including
- Improved memory management: The JVM’s automatic garbage collection frees developers from the responsibility of manually managing memory allocation and deallocation, reducing the likelihood of memory leaks and other memory-related errors.
- Increased developer productivity: By removing the need for manual memory management, garbage collection allows developers to focus on writing code, increasing their productivity and reducing the time required to develop software.
- Improved performance: The JVM’s garbage collector optimizes memory usage by reclaiming memory occupied by objects that are no longer needed, reducing fragmentation and improving performance.
- Reduced risk of memory-related errors: By automatically freeing memory occupied by unused objects, garbage collection reduces the risk of memory-related errors, such as buffer overflows, null pointer exceptions, and memory leaks.
- Improved code readability and maintainability: Garbage collection eliminates the need for manual memory management, reducing the amount of low-level code in a program and improving the readability and maintainability of the code.
- Better memory utilization: The JVM’s garbage collector optimizes memory utilization by reclaiming memory occupied by unused objects, making more memory available to the program and improving its overall performance.
- Portable code: Garbage collection is a fundamental feature of the Java platform, and its behavior is consistent across all JVMs. This means that Java programs written with garbage collection in mind are portable, reducing the effort required to develop and maintain code on different platforms.
Example
Here’s an example to demonstrate garbage collection in Java:
class Person {
String name;
public Person(String name) {
this.name = name;
}
}
public class Main {
public static void main(String[] args) {
Person p1 = new Person("John");
Person p2 = new Person("Jane");
p1 = null; // p1 is now eligible for garbage collection
}
}
Explanation
In the example above, we create two instances of the Person class, p1 and p2. After assigning p1 to null, it is no longer reachable and eligible for garbage collection. The JVM will eventually run the garbage collector to reclaim the memory occupied by p1.
Summary
Garbage collection in java is an important feature of Java that helps to simplify memory management and reduce the risk of memory leaks. However, it is a non-deterministic process and should not be relied upon to control the timing of memory cleanup in programs. It provides several benefits, including improved memory management, increased developer productivity, reduced risk of memory-related errors, and improved performance. Effective use of garbage collection can help developers to write high-quality, memory-efficient code that is easy to maintain and portable across different platforms.
It’s also worth noting that Java provides methods such as System.gc() and Runtime.gc() that allow developers to request garbage collection, but these methods only provide a hint to the JVM and do not guarantee that garbage collection will occur. Additionally, relying on these methods to control the timing of garbage collection can negatively impact performance.
FAQs Related to Garbage Collection:
1. What is garbage collection in Java?
Garbage collection is a mechanism in Java to automatically free up memory occupied by objects that are no longer being used by the program.
2. How does garbage collection work in Java?
The Java virtual machine periodically identifies objects in memory that are no longer being used by the program, and frees up the memory occupied by those objects.
3. Who is responsible for garbage collection in Java?
The Java virtual machine is responsible for garbage collection in Java.
4. Can garbage collection be forced in Java?
Yes, garbage collection can be requested by calling System.gc(), but it is not guaranteed to run immediately.
5. Can you control when an object becomes eligible for garbage collection?
No, the Java virtual machine determines when an object becomes eligible for garbage collection, based on whether it can be reached by any live threads or objects in the program.
6: What is the difference between finalize and destroy methods in Java?
There is no "destroy" method in Java, but there is a "finalize" method. The finalize method is called just before an object is garbage collected, and can be overridden by the programmer to perform cleanup operations. However, the finalized method is not guaranteed to be called and is generally considered a dangerous and unreliable method for resource cleanup.