In java, garbage means unreferenced objects.
Garbage Collection is process of reclaiming the runtime unused memory automatically. In other words, it is a way to destroy the unused objects.
To do so, we were using free() function in C language and delete() in C++. But, in java it is performed automatically. So, java provides better memory management.

Table of Contents

Advantage of Garbage Collection

• It makes java memory efficient because garbage collector removes the unreferenced objects from heap memory.
• It is automatically done by the garbage collector(a part of JVM) so we don’t need to make extra efforts.

How can an object be unreferenced?

There are many ways:
• By nulling the reference
• By assigning a reference to another
• By anonymous object etc.

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1) By nulling a reference:

1. Employee e=new Employee();
2. e=null;

2) By assigning a reference to another:

1. Employee e1=new Employee();
2. Employee e2=new Employee();
3. e1=e2;//now the first object referred by e1 is available for garbage collection

3) By anonymous object:

1. new Employee();

finalize() method

The finalize() method is invoked each time before the object is garbage collected. This method can be used to perform cleanup processing. This method is defined in Object class as:
1. protected void finalize(){}
Note: The Garbage collector of JVM collects only those objects that are created by new keyword. So if you have created any object without new, you can use finalize method to perform cleanup processing (destroying remaining objects).

gc() method

The gc() method is used to invoke the garbage collector to perform cleanup processing. The gc() is found in System and Runtime classes.
1. public static void gc(){}
Note: Garbage collection is performed by a daemon thread called Garbage Collector(GC). This thread calls the finalize() method before object is garbage collected.

Simple Example of garbage collection in java

1. public class TestGarbage1{ 
2. public void finalize(){System.out.println("object is garbage collected");} 
3. public static void main(String args[]){ 
4. TestGarbage1 s1=new TestGarbage1(); 
5. TestGarbage1 s2=new TestGarbage1(); 
6. s1=null; 
7. s2=null; 
8. System.gc(); 
9. } 
10. }

object is garbage collected
object is garbage collected
Note: Neither finalization nor garbage collection is guaranteed

Java Runtime class

Java Runtime class is used to interact with java runtime environment. Java Runtime class provides methods to execute a process, invoke GC, get total and free memory etc. There is only one instance of java.lang.Runtime class is available for one java application.
The Runtime.getRuntime() method returns the singleton instance of Runtime class.

Important methods of Java Runtime class

No.	Method	Description
1)	public static Runtime getRuntime()	returns the instance of Runtime class.
2)	public void exit(int status)	terminates the current virtual machine.
3)	public void addShutdownHook(Thread hook)	registers new hook thread.
4)	public Process exec(String command)throws IOException	executes given command in a separate process.
5)	public int availableProcessors()	returns no. of available processors.
6)	public long freeMemory()	returns amount of free memory in JVM.
7)	public long totalMemory()	returns amount of total memory in JVM.

Java Runtime exec() method

1. public class Runtime1{ 
2. public static void main(String args[])throws Exception{ 
3. Runtime.getRuntime().exec("notepad");//will open a new notepad 
4. } 
5. }

How to shutdown system in Java

You can use shutdown -s command to shutdown system. For windows OS, you need to provide full path of shutdown command e.g. c:\\Windows\\System32\\shutdown.
Here you can use -s switch to shutdown system, -r switch to restart system and -t switch to specify time delay.

1. public class Runtime2{ 
2. public static void main(String args[])throws Exception{ 
3. Runtime.getRuntime().exec("shutdown -s -t 0"); 
4. } 
5. }

How to shutdown windows system in Java

1. public class Runtime2{ 
2. public static void main(String args[])throws Exception{ 
3. Runtime.getRuntime().exec("c:\\Windows\\System32\\shutdown -s -t 0"); 
4. } 
5. }

How to restart system in Java

1. public class Runtime3{ 
2. public static void main(String args[])throws Exception{ 
3. Runtime.getRuntime().exec("shutdown -r -t 0"); 
4. } 
5. }

Java Runtime availableProcessors()

1. public class Runtime4{ 
2. public static void main(String args[])throws Exception{ 
3. System.out.println(Runtime.getRuntime().availableProcessors()); 
4. } 
5. }

Java Runtime freeMemory() and totalMemory() method

In the given program, after creating 10000 instance, free memory will be less than the previous free memory. But after gc() call, you will get more free memory.

1. public class MemoryTest{ 
2. public static void main(String args[])throws Exception{ 
3. Runtime r=Runtime.getRuntime(); 
4. System.out.println("Total Memory: "+r.totalMemory()); 
5. System.out.println("Free Memory: "+r.freeMemory()); 
6. 
7. for(int i=0;i<10000;i++){ 
8. new MemoryTest(); 
9. } 
10. System.out.println("After creating 10000 instance, Free Memory: "+r.freeMemory()); 
11. System.gc(); 
12. System.out.println("After gc(), Free Memory: "+r.freeMemory()); 
13. } 
14. }

Total Memory: 100139008
Free Memory: 99474824
After creating 10000 instance, Free Memory: 99310552
After gc(), Free Memory: 100182832

So, this brings us to the end of blog. This Tecklearn ‘Java Garbage Collection and Java Runtime Class’ blog helps you with commonly asked questions if you are looking out for a job in Java Programming. If you wish to learn Java and build a career Java Programming domain, then check out our interactive, Java and JEE Training, that comes with 24*7 support to guide you throughout your learning period. Please find the link for course details:

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