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Garbage Collection in Java | Types | How It works | Example
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Garbage Collector Internal working
1.What is garbage collection
It is the process of removing unused object from java heap area and making heap space empty for use
In c and c++ programmer is responsible for destroy of created object
But in java it is given to jvm
We can call garbage collection using
Finalise method
But it will not give guarantee that it will call the garbage collector.
When you run your program on jvm . Objects get created in heap areas.
Some of them are used in application for long time but some are for short period
Those short period objects occupy space from the heap area.
Below scenarios can create objects for short time period
Test t1= new Test()
t1=null
Test t1= new Test();
Test t2=new Test();
t1=t2;
return (new Test)
How does Garbage Collection Work in Java?
It is an automatic process handled by JVM. it specify all its standard and according to that it gets called
It scan all heap memory and unreachable object gets garbage collected
Phases of Garbage Collection in Java
A standard Garbage Collection implementation involves three phases
1.Mark all referenced objects
2. Sweep dead Objects
All the non referenced objects gets destroyed.
3.Compact remaining space by arranging objects
To make all remaining objects in continuous formats
Heap Memory is divided into below area’s
Young generation
Eden space - all new objects start here, and initial memory is allocated to them
Survivor spaces (s0 and s1) - objects are moved here from Eden after surviving one garbage collection cycle.
You can use the -Xmn flag to set the size of the Young Generation.
Old Generation
Objects that are long-lived are eventually moved from the Young Generation to the Old Generation. This is also known as Tenured Generation, and contains objects that have remained in the survivor spaces for a long time
When objects are garbage collected from the Old Generation, it is a major garbage collection event.
You can use the -Xms and -Xmx flags to set the size of the initial and maximum size of the Heap memory.
Permanent Generation
It store meta information about classes and methods used while running applications
You can use the -XX:PermGen and -XX:MaxPermGen flags to set the initial and maximum size of the Permanent Generation.
Also String constant pool is reciting in Perm gen memory area
MetaSpace
Starting with Java 8, the MetaSpace memory space replaces the PermGen space. The implementation differs from the PermGen and this space of the heap is now automatically resized.
It is used to avoid memory out of exception
Type of GC
Serial GC
only one thread is used to collect all garbage from all phase of memory
It is stop world event ,where all threads in application stops and only GC thread runs
It is very difficult for application which has low latency
Eg. Share market
2.Parallel GC
Where multiple threads simultaneously collect garbage.
This is the default implementation of GC in the JVM and is also known as Throughput Collector
3.CMS (Concurrent Mark Sweep) GC
CMS runs concurrently alongside application processes to minimise “stop the world” events.
Due to both application and GC threads running simultaneously it consumes more memory.
4. G1 (Garbage First) GC
It is parallel and concurrent like CMS, but it works quite differently under the hood compared to the older garbage collectors.
It divide your heap into multiple similar blocks according to your heap memory and scan parallel those blocks
After the mark phase is completed, G1 knows which regions contain the most garbage objects. If the user is interested in minimal pause times, G1 can choose to evacuate only a few regions. If the user is not worried about pause times or has stated a fairly large pause-time goal, G1 might choose to include more regions.
Since G1GC identifies the regions with the most garbage and performs garbage collection on that region first, it is called Garbage First
1.What is garbage collection
It is the process of removing unused object from java heap area and making heap space empty for use
In c and c++ programmer is responsible for destroy of created object
But in java it is given to jvm
We can call garbage collection using
Finalise method
But it will not give guarantee that it will call the garbage collector.
When you run your program on jvm . Objects get created in heap areas.
Some of them are used in application for long time but some are for short period
Those short period objects occupy space from the heap area.
Below scenarios can create objects for short time period
Test t1= new Test()
t1=null
Test t1= new Test();
Test t2=new Test();
t1=t2;
return (new Test)
How does Garbage Collection Work in Java?
It is an automatic process handled by JVM. it specify all its standard and according to that it gets called
It scan all heap memory and unreachable object gets garbage collected
Phases of Garbage Collection in Java
A standard Garbage Collection implementation involves three phases
1.Mark all referenced objects
2. Sweep dead Objects
All the non referenced objects gets destroyed.
3.Compact remaining space by arranging objects
To make all remaining objects in continuous formats
Heap Memory is divided into below area’s
Young generation
Eden space - all new objects start here, and initial memory is allocated to them
Survivor spaces (s0 and s1) - objects are moved here from Eden after surviving one garbage collection cycle.
You can use the -Xmn flag to set the size of the Young Generation.
Old Generation
Objects that are long-lived are eventually moved from the Young Generation to the Old Generation. This is also known as Tenured Generation, and contains objects that have remained in the survivor spaces for a long time
When objects are garbage collected from the Old Generation, it is a major garbage collection event.
You can use the -Xms and -Xmx flags to set the size of the initial and maximum size of the Heap memory.
Permanent Generation
It store meta information about classes and methods used while running applications
You can use the -XX:PermGen and -XX:MaxPermGen flags to set the initial and maximum size of the Permanent Generation.
Also String constant pool is reciting in Perm gen memory area
MetaSpace
Starting with Java 8, the MetaSpace memory space replaces the PermGen space. The implementation differs from the PermGen and this space of the heap is now automatically resized.
It is used to avoid memory out of exception
Type of GC
Serial GC
only one thread is used to collect all garbage from all phase of memory
It is stop world event ,where all threads in application stops and only GC thread runs
It is very difficult for application which has low latency
Eg. Share market
2.Parallel GC
Where multiple threads simultaneously collect garbage.
This is the default implementation of GC in the JVM and is also known as Throughput Collector
3.CMS (Concurrent Mark Sweep) GC
CMS runs concurrently alongside application processes to minimise “stop the world” events.
Due to both application and GC threads running simultaneously it consumes more memory.
4. G1 (Garbage First) GC
It is parallel and concurrent like CMS, but it works quite differently under the hood compared to the older garbage collectors.
It divide your heap into multiple similar blocks according to your heap memory and scan parallel those blocks
After the mark phase is completed, G1 knows which regions contain the most garbage objects. If the user is interested in minimal pause times, G1 can choose to evacuate only a few regions. If the user is not worried about pause times or has stated a fairly large pause-time goal, G1 might choose to include more regions.
Since G1GC identifies the regions with the most garbage and performs garbage collection on that region first, it is called Garbage First