Java Memory Management

How memory is managed in java?

Java supports automatic garbage collection, Java's garbage collector runs in frequently to recycle or clean up unused objects.

Java also provides the provision to run garbage collector on demand by invoking gc method which is available in Runtime object.

Sometimes we may want to know how much object size in heap and how much size left in heap etc. Java has totalMemory and freeMemory methods to get these information.

Memory management supports to check code for efficiency or how many objects of certain type can be instantiated etc.

Java Programming Performs Garbage Collection

This simple java program illustrates to trigger the garbage collection manually.

freeMemory method in Runtime class is used to get the available memory size in JVM (Java Virtual MAchine).

gc method is used to perform the garbage collection.

import java.util.*;

public class GarbageCollectionTest {
	public static void main(String[] args){
		Runtime runtime = Runtime.getRuntime();
		long freeJVMMemory = runtime.freeMemory();
		System.out.println("JVM Free Memory Before Garbage Collection: " +freeJVMMemory);
		// Performs garbage collection.
		runtime.gc();
		freeJVMMemory = runtime.freeMemory();
		System.out.println("JVM Free Memory After Garbage Collection: " +freeJVMMemory);
	}
}

Output:

$ javac GarbageCollectionTest.java
$ java GarbageCollectionTest 
JVM Free Memory Before Garbage Collection: 125535480
JVM Free Memory After Garbage Collection: 126594096

How to do clean up unused objects in java manually?

gc() method is available in Runtime object to manually recycle or clean up unused objects in java.

Java program with running garbage collection on demand

import java.util.*;                                                             
class MemoryManagement {                                                        
    public static void main(String args[]) {                                    
        Runtime runtime = Runtime.getRuntime();                                 
        Integer integerList[] = new Integer[2000];    
        // Getting total memory in heap on machine                          
        System.out.println("Total Memory: " + runtime.totalMemory());  
        // Getting free memory in heap on machine         
        System.out.println("Initial Free Memory: " + runtime.freeMemory());  
        // Manually running garbage collector   
        runtime.gc();                                                           
        long memoryBefore = runtime.freeMemory();                               
        System.out.println("Free memory after Garbage Collection: " + memoryBefore);
        Random random = new Random();    
        // Alocating memory for each integer elements in array                                       
        for(int i =0; i<2000; i++) {                         
            //Generates random integer number for each array elements                   
            integerList[i] = new Integer(random.nextInt());                     
        }                              
        // Gets free memory after array elements allocation
        long memoryAfter  = runtime.freeMemory();                               
        System.out.println("Free memory after allocation:  " + memoryAfter);  
        // Difference to get allocation size before and after allocated for integer array elements.  
        System.out.println("Memory is used for allocation:  " + (memoryBefore-memoryAfter));
        // Deallocating array elements
        for(int i =0; i<2000; i++) {                                            
            integerList[i] = null;                                              
        }                                                                       
        //Garbage collection                                                    
        runtime.gc();                 
        // After deallocation free memory size
        System.out.println("Free memory after deallocation: " + runtime.freeMemory());
    }                                                                           
}  

Output:

$ java MemoryManagement
Total Memory: 247463936
Initial Free Memory: 246163688
Free memory after Garbage Collection: 247194624
Free memory after allocation:  245894320
Memory is used for allocation:  1300304
Free memory after deallocation: 245891576

What is finalize method in java?

Sometimes object may need to perform some action when it's destroyed. suppose object is holding some non java resource such as a file handle or window character font, so object might ensure these resources are freed before object is destroyed.
Java provides finalize method to handle some action before an object is destroyed.

protected void finalize() {
//code to release resources which object holds.
}

finalize() method will be invoke when an object is just about to be destroyed by garbage collector in java.
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