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Memory management and garbage collection

From: Java Essential Training

Video: Memory management and garbage collection

One of the great charms of the Java programing language is that even though it's a compiled language, it manages memory for you, unlike languages like C, C++, Pascal, and others. You as the programmer don't have to explicitly allocate and de-allocate memory whenever you create an object. The Garbage Collector is a major feature of the Java Virtual Machine. I am going to describe how the Garbage Collector works and how little attention you as the programmer need to give it. When you create a variable that references a complex object, the object itself is stored in an area of memory called heap memory.

Memory management and garbage collection

One of the great charms of the Java programing language is that even though it's a compiled language, it manages memory for you, unlike languages like C, C++, Pascal, and others. You as the programmer don't have to explicitly allocate and de-allocate memory whenever you create an object. The Garbage Collector is a major feature of the Java Virtual Machine. I am going to describe how the Garbage Collector works and how little attention you as the programmer need to give it. When you create a variable that references a complex object, the object itself is stored in an area of memory called heap memory.

There are two types of memory; stack and heap. The stack memory is somewhat faster; heap a little bit slower, but heap is more dynamic. When you create primitive variables, depending on their context, they might be stored in stack or heap, but complex objects are always stored in heap. As long as any variable references an object in memory, that object will be retained, it won't be eligible for garbage collection. So if you create a variable and it points to a string, a date, an array, a collection, or any other complex object, as long as that variable can be addressed in your code, the object will be available.

When all references to the object expire, that is when you can't get to the object through your code anymore, then that object is eligible for garbage collection. So when do references expire? There are two models that you can look at. First of all, when you declare a variable that's local to a function or to a code block, as soon as the function or code block is done executing, that variable is no longer available to you, and so it has expired and is eligible for garbage collection. Within this function there are two scopes.

Variables declared directly within the function are available as long as the function is executing. So the variable in function would be available until the function is done, and then it would expire. I've spoken previously about the fact that in Java, scope is also defined by code blocks. So in the second part of the code, in the for loop, I am declaring an integer as a part of the for loop, the name of the variable as i, and as soon as that for loop is done executing, that variable is no longer in scope and it's eligible for garbage collection.

For variables that are more persistent, such as fields within objects, you can explicitly expire them by setting their value to null, N-U-L-L. So for example, here is string named expireMe, and if I want to make it eligible for garbage collection, I simply set its value to null. The object in memory can't be reached in my code anymore and therefore it's eligible for garbage collection. So when does garbage collection happen? The Garbage Collector runs in its own thread, so it doesn't affect your applications.

As long as the system on which your application is running has adequate resources, you shouldn't notice that the Collector is running. And the Garbage Collector has incredible autonomy. The Java Virtual Machine runs the Garbage Collector when it thinks it needs to, and when the Garbage Collector identifies an object that no longer has any references, it has the option of either destroying it and reclaiming the memory or not. It has its own algorithm, its own logic, for figuring out how to manage the memory, and it's not up to you.

You as a programmer cannot force garbage collection in Java. There are methods available in the language, such as System.gc() and Runtime.gc(), gc stands for Garbage Collector. These are methods that send messages to the JVM, that say it might be a good idea to collect garbage right now, but there is no guarantee that it will happen. And in fact, many Java developers recommend not using these functions. It can mislead you into thinking that you are getting all your memory back right away, but that's not necessarily what's going to happen.

If you're running a complex application and you are challenging the available memory on the system, you will know, because the Java Virtual Machine will throw an error called OutOfMemoryError, and you will see these errors happen in more complex applications where an application is running on low memory devices or systems. You can manage the memory of the Java Virtual Machine by setting certain parameters when you start up an application. You can also tune your application by minimizing the number of objects that you create.

As you've seen in many of the examples in this course, there are different ways of architecting Java code, so that you can create a whole bunch of objects, or you can reuse objects that you've already created. In general, it's better to reuse. If you're concerned about memory usage, you can use these methods to find out how much memory is available: Runtime.maxMemory(), Runtime. totalMemory(), and Runtime.freeMemory(). Take a look at the Java docs for these methods to find out what information they are giving you. And you can use command line options to manage the amount of available heap memory.

You can manage the memory in three areas; in the initial heap size, in the maximum heap size, and in the heap size for what are called young generation objects; objects that have just been created. If you want more information about memory management, take a look at the documentation for the Java programing language on the Oracle website. You will find that there is an enormous amount of information available. But when you're writing your first simple Java applications just know that the Garbage Collector is highly automated and as long as you are not running out of memory, you might not need to worry about it.

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This video is part of

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Java Essential Training

71 video lessons · 72217 viewers

David Gassner
Author

 
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  1. 10m 8s
    1. Welcome
      1m 3s
    2. Is this course for you?
      5m 35s
    3. Using the exercise files
      3m 30s
  2. 31m 24s
    1. The history of Java
      5m 19s
    2. Java compilation and syntax
      8m 54s
    3. Understanding the principles of Java
      8m 28s
    4. Choosing a development environment
      8m 43s
  3. 19m 5s
    1. Installing Java on Windows
      6m 42s
    2. Installing Eclipse on Windows
      3m 19s
    3. Exploring Java on Mac OS X Leopard and Snow Leopard
      2m 27s
    4. Installing Java on Mac OS X Lion
      3m 27s
    5. Installing Eclipse on Mac OS X
      3m 10s
  4. 46m 10s
    1. Creating a Hello World application
      11m 7s
    2. Exploring the Eclipse IDE
      8m 55s
    3. Compiling and running from the command line
      8m 2s
    4. Passing arguments to the application
      8m 17s
    5. Using the Java API documentation
      4m 5s
    6. Memory management and garbage collection
      5m 44s
  5. 58m 57s
    1. Everything is an object
      5m 59s
    2. Declaring and initializing variables
      9m 15s
    3. Working with numbers
      8m 32s
    4. Converting numeric values
      6m 40s
    5. Understanding operators
      7m 58s
    6. Working with character values
      5m 14s
    7. Working with boolean values
      5m 13s
    8. Outputting primitive values as strings
      5m 33s
    9. Creating a simple calculator application
      4m 33s
  6. 53m 40s
    1. Writing conditional code
      5m 35s
    2. Using the switch statement
      8m 50s
    3. Repeating code blocks with loops
      7m 35s
    4. Creating reusable code with methods
      6m 31s
    5. Declaring methods with arguments
      5m 41s
    6. Overloading method names with different signatures
      5m 53s
    7. Passing arguments by reference or by value
      5m 35s
    8. Creating a more complex calculator application
      8m 0s
  7. 20m 30s
    1. Using the String class
      5m 44s
    2. Building strings with StringBuilder
      3m 34s
    3. Parsing string values
      3m 19s
    4. Working with date values
      7m 53s
  8. 20m 44s
    1. Understanding compile-time vs. runtime errors
      4m 5s
    2. Handling exceptions with try/catch
      4m 55s
    3. Throwing exceptions in methods
      2m 50s
    4. Using the debugger
      8m 54s
  9. 32m 22s
    1. Using simple arrays
      4m 47s
    2. Using two-dimensional arrays
      6m 17s
    3. Managing resizable arrays with ArrayList
      7m 14s
    4. Managing unordered data with HashMap
      6m 5s
    5. Looping through collections with iterators
      7m 59s
  10. 52m 2s
    1. Understanding encapsulation
      5m 59s
    2. Creating and instantiating custom classes
      8m 8s
    3. Organizing classes with packages
      6m 47s
    4. Creating and using instance methods
      6m 52s
    5. Storing data in instance variables
      6m 56s
    6. Using constructor methods
      5m 40s
    7. Managing instance data with getter and setter methods
      8m 26s
    8. Using class variables and Enum classes
      3m 14s
  11. 41m 15s
    1. Understanding inheritance and polymorphism
      9m 12s
    2. Extending custom classes
      9m 1s
    3. Overriding superclass methods
      3m 8s
    4. Casting subclass objects
      5m 3s
    5. Understanding interfaces and implementing classes
      4m 2s
    6. Creating your own interfaces
      4m 14s
    7. Using abstract classes and methods
      6m 35s
  12. 32m 17s
    1. Managing files with the core class library
      7m 46s
    2. Managing files with Apache Commons FileUtils
      7m 32s
    3. Reading a text file from a networked resource
      7m 52s
    4. Parsing an XML file with DOM
      9m 7s
  13. 17m 39s
    1. Creating your own JAR files
      4m 54s
    2. Understanding the classpath
      5m 2s
    3. Documenting code with Javadoc
      7m 43s
  14. 47s
    1. Goodbye
      47s

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