Garbage Collection in Java

Memory management in Java

Java's memory management is highly appealing to programmers as it relieves them from the burden of manual memory allocation and deallocation. The automatic garbage collector in Java efficiently reclaims memory for reuse, eliminating the need for explicit memory management. When objects are created, they automatically acquire the required memory, and when they are no longer needed, the garbage collection process identifies them as garbage and reclaims the memory. While garbage collection resolves many memory-related concerns, it can sometimes introduce significant performance overhead.

How can an object be unreferenced?

When a Java program is executing, it often creates new objects like Strings and Files. However, as the program progresses, certain objects become unused and serve no purpose within the application. Consider the following code snippet as an example:

public void processFiles(List<String> fileNames) { for (String fileName : fileNames) { File file = new File(fileName); // Process the file // ... } }

In the above code, a list of file names is passed to the processFiles method. Within the method, a new File object is created for each file name in the list. However, once the file is processed, the File objects become unnecessary.

In scenarios like this, Java's garbage collector plays a crucial role. It periodically identifies objects that are no longer reachable and reclaims the memory they occupy. In our example, once the File objects are no longer referenced, they become eligible for garbage collection.

How Java garbage collection works

Here's a detailed explanation of how Java garbage collection works:

Object Lifecycle

In Java, objects are created dynamically using the new keyword. Once an object is created, it resides in the heap memory. The object remains in memory as long as it is reachable or referenced by active variables or data structures. When an object is no longer reachable, it becomes eligible for garbage collection.

Garbage Collector

The garbage collector is responsible for reclaiming memory occupied by unreachable objects. It periodically examines objects in memory to identify those that are no longer reachable and frees up the associated memory. The garbage collector runs in the background, transparently managing memory without developer intervention.

Reachability and Marking

The garbage collector determines object reachability by using a process called marking. It starts with a set of root objects (e.g., global variables, method local variables, and reference chains). It marks all objects directly or indirectly reachable from the root set as live or reachable. Any objects not marked as live are considered garbage.

Garbage Collection Algorithms

Java employs various garbage collection algorithms to perform the memory reclamation process. Some commonly used algorithms include:

  1. Mark-and-Sweep: This algorithm marks live objects during the marking phase and then sweeps through the memory, reclaiming memory occupied by unreachable objects.
  2. Copying: This algorithm divides memory into two halves. Objects are initially allocated in one half, and when garbage collection occurs, live objects are copied to the other half while discarding unreachable objects.
  3. Generational: This algorithm divides objects into different generations based on their age. Younger objects that are more likely to become garbage are collected more frequently, while older objects are collected less frequently.

System.gc() and finalize()

Java provides the System.gc() method as a hint to the garbage collector to initiate garbage collection. However, the actual invocation and behavior of garbage collection are implementation-dependent, and there is no guarantee of immediate execution.

The finalize() method is a mechanism provided by Java for objects to perform cleanup operations before being garbage collected. It is called by the garbage collector before reclaiming an object. However, its usage is generally discouraged due to uncertainty regarding when it will be invoked.

public class GarbageCollectionExample { public static void main(String[] args) { // Creating objects MyClass obj1 = new MyClass(); MyClass obj2 = new MyClass(); // Setting obj1 reference to null, making it eligible for garbage collection obj1 = null; // Requesting garbage collection (not guaranteed to execute immediately) System.gc(); // Rest of the code } } class MyClass { // Class members protected void finalize() { // Cleanup operations before object is garbage collected } }

In the above example, when obj1 is set to null, it becomes unreachable and eligible for garbage collection. The System.gc() method is then invoked to suggest garbage collection, although the actual execution timing is implementation-dependent. The finalize() method in MyClass can be overridden to perform any necessary cleanup operations before the object is garbage collected.

How to force garbage collection in Java?

The invocation of System.gc() in Java is intended as a suggestion to the garbage collector, indicating that a collection is desired. However, it is important to note that the actual execution of garbage collection is determined by the Java Virtual Machine (JVM) and is not guaranteed to occur immediately.

The JVM internally employs algorithms and heuristics to determine the optimal timing for garbage collection based on factors such as available memory, system load, and garbage collection configuration. When System.gc() is called, it serves as a request to the JVM, but the JVM retains the discretion to ignore the request and defer garbage collection to a more suitable time.

It is crucial to understand that the decision of when to perform garbage collection is ultimately the responsibility of the JVM, which aims to balance memory management efficiency with overall application performance. Relying on explicit calls to System.gc() for fine-tuning memory management is generally discouraged, as it can interfere with the JVM's built-in garbage collection mechanisms and lead to unpredictable behavior.

Is it good practice to call Garbage Collector manually?

No, manually invoking the Garbage Collector is generally considered bad practice. The garbage collector implemented in the Java Virtual Machine (JVM) is equipped with sophisticated algorithms and logic to determine the optimal timing and approach for memory cleanup. Adjusting the garbage collector requires a deep understanding of its inner workings and mechanisms. Simply adding a System.gc() statement in the code is unlikely to yield significant improvements and can potentially have adverse effects.

Some individuals may attempt to nullify object references or use the System.gc() method to explicitly free up memory. While setting references to null is relatively harmless, invoking the System.gc() method can have severe repercussions on system performance and is strongly discouraged.

The JVM's garbage collector is designed to autonomously manage memory and initiate garbage collection when necessary. It employs intricate strategies to optimize memory utilization and application performance. Attempting to micromanage memory by manually triggering the garbage collector can disrupt this delicate balance and lead to undesirable consequences.

It is advisable to trust the JVM's built-in memory management mechanisms and allow the garbage collector to function autonomously. Focus on writing efficient and clean code, and let the JVM handle memory management effectively.

Advantage/Disadvantages of Garbage Collection

  1. Automatic deallocation allows a programmer not to worry about memory management, increasing write ability of a system, and decreasing development time and costs.
  2. It is automatically done by the garbage collector(a part of JVM) so we don't need to make extra efforts.
  3. Garbage collection is a language requirement for functional languages, which cannot use a stack-based environment because of unpredictable execution patterns.
  4. Explicit management introduces possibilities for making errors in memory management - for example, memory leaks. Thus, explicit deallocation decreases reliability.

When Is The Object Eligible For Garbage Collection?

Generally, an object becomes eligible for garbage collection in Java on following cases:

  1. Any instances that cannot be reached by a live thread.
  2. Circularly referenced instances that cannot be reached by any other instances.
  3. If an object has only lived weak references via WeakHashMap it will be eligible for garbage collection.
  4. The object is created inside a block and reference goes out scope once control exit that block.

Does GC guarantee that a program will not run out of memory?

The process of garbage collection (GC) in Java does not provide an absolute assurance that a program will never encounter memory exhaustion. It is imperative for developers to take responsibility for managing objects that are no longer in use and ensure they are no longer referenced within the application. By releasing references to such objects, the garbage collector can effectively perform its task and reclaim the memory consumed by these unused objects.

Does assigning objects to null in Java impact garbage collection?

Not necessarily. the eligibility of an object for garbage collection in Java is determined by the absence of any live threads that maintain references to the object. Explicitly setting a reference to null, as opposed to allowing the variable to naturally go out of scope, does not inherently assist the garbage collector unless the object being referenced is of substantial size.

The primary criterion for an object to be collected by the garbage collector is the absence of any active threads that retain direct or indirect references to the object. In this context, explicitly assigning null to a reference does not directly influence the garbage collection process, as the key factor is the lack of live threads maintaining access to the object.


Java's garbage collection simplifies memory management and reduces the risk of memory-related errors. However, understanding garbage collection principles and best practices can help optimize memory usage and improve the performance of Java applications.