What is Garbage Collection (GC)?
• The Java virtual machine's heap stores all objects created by a running Java application.
• Objects are created by the program through new keyword, but never freed explicitly by the program
> No need to call free().
• Garbage collection is the process of automatically freeing objects that are no longer needed
• An object is determined to be “no longer needed” when there is no other object referencing to it
> Each object has a reference counter - when it becomes 0, it means there is no other object referencing to it
Why Garbage Collection (GC)?
Advantages of GC
• Programmer is free from memory management
> Less error prone code
• System cannot crash due to memory management
> More reliable application
> Memory-leak is still possible, however - you can use memory profiler to find out where memory-leaking code is located
Disadvantages of GC
• GC could add overhead
> Many GC schemes are focused on minimizing GC overhead
• GC can occur in an non-deterministic way
> Real-time Java addresses this
When and How Garbage Collection (GC) Occur?
• JVM performs GC when it determines the amount of free heap space is below a threshold
> This threshold can be set when a Java application is run
How Does JVM Perform GC?
• The garbage collector must somehow determine which objects are no longer referenced and make available the heap space occupied by such unreferenced objects.
• The simplest and most crude scheme is to keep reference counter to each object
• There are many different schemes - years of research
Java API
GC Related Java API
• finalize() method in Object class
> Called by the garbage collector on an object when garbage collection determines that there are no more references to the object.
• gc() method in System class
> Runs the garbage collector.
> Calling the gc method suggests that the Java Virtual Machine expend effort toward recycling unused objects in order to make the memory they currently occupy available for quick reuse.
> When control returns from the method call, the Java Virtual Machine has made
• The Java virtual machine's heap stores all objects created by a running Java application.
• Objects are created by the program through new keyword, but never freed explicitly by the program
> No need to call free().
• Garbage collection is the process of automatically freeing objects that are no longer needed
• An object is determined to be “no longer needed” when there is no other object referencing to it
> Each object has a reference counter - when it becomes 0, it means there is no other object referencing to it
Why Garbage Collection (GC)?
Advantages of GC
• Programmer is free from memory management
> Less error prone code
• System cannot crash due to memory management
> More reliable application
> Memory-leak is still possible, however - you can use memory profiler to find out where memory-leaking code is located
Disadvantages of GC
• GC could add overhead
> Many GC schemes are focused on minimizing GC overhead
• GC can occur in an non-deterministic way
> Real-time Java addresses this
When and How Garbage Collection (GC) Occur?
• JVM performs GC when it determines the amount of free heap space is below a threshold
> This threshold can be set when a Java application is run
How Does JVM Perform GC?
• The garbage collector must somehow determine which objects are no longer referenced and make available the heap space occupied by such unreferenced objects.
• The simplest and most crude scheme is to keep reference counter to each object
• There are many different schemes - years of research
Java API
GC Related Java API
• finalize() method in Object class
> Called by the garbage collector on an object when garbage collection determines that there are no more references to the object.
• gc() method in System class
> Runs the garbage collector.
> Calling the gc method suggests that the Java Virtual Machine expend effort toward recycling unused objects in order to make the memory they currently occupy available for quick reuse.
> When control returns from the method call, the Java Virtual Machine has made
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