Java heap size (memory) allocation
The amount of memory that a Java program is allowed to use depends on its environment. Java Standard library provides the following methods in the runtime class.
The totalMemory() returns the total amount of memory in the JVM. The value returned by this method may vary over time, depending on the host environment.
The freeMemory() returns the amount of free memory in the JVM. Calling the gc method may result in increasing the value returned by freeMemory.
The maxMemory() returns the maximum amount of memory that the JVM will attempt to use. If there is no inherent limit then the value Long.MAX_VALUE will be returned.
What are the Xms and Xmx parameters?
The flag -Xmx < memory > specifies the maximum memory allocation pool for a Java Virtual Machine (JVM), while -Xms< memory > specifies the initial memory allocation pool. The memory flag can also be specified in multiple sizes, such as kilobytes, megabytes, and so on.
Starting a JVM like below will start it with 256MB of memory, and will allow the process to use up to 2048MB of memory:
The following program will output jvm options and the used, free, total and maximum memory available in jvm.
- Java Interview Questions-Core Faq - 1
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- Important features of Java
- What is the difference between JDK and JRE?
- What gives Java its 'write once and run anywhere' nature?
- What is JVM and is it platform independent?
- What is Just-In-Time (JIT) compiler?
- What is the garbage collector in Java?
- What is NullPointerException in Java
- Difference between Stack and Heap memory in Java
- What is numeric promotion?
- Why do we need Generic Types in Java?
- What does it mean to be static in Java?
- What are final variables in Java?
- How Do Annotations Work in Java?
- How do I use the ternary operator in Java?
- What is instanceof keyword in Java?
- How ClassLoader Works in Java?
- What are fail-safe and fail-fast Iterators in Java
- What are method references?
- "Cannot Find Symbol" compile error
- Difference between system.gc() and runtime.gc()
- How to convert TimeStamp to Date in Java?
- Does garbage collection guarantee that a program will not run out of memory?
- How setting an Object to null help Garbage Collection?
- How do objects become eligible for garbage collection?
- How to calculate date difference in Java
- Difference between Path and Classpath
- Is Java "pass-by-reference" or "pass-by-value"?
- Difference between static and nonstatic methods java
- Why Java does not support pointers?
- What is package in Java?
- What are wrapper classes?
- What is singleton class in Java?
- Difference between Java Local Variable, Instance Variable and a Class Variable?
- Can a top level class be private or protected in java
- Are Polymorphism , Overloading and Overriding similar concepts?
- How to Use Locks in Java
- Why Multiple Inheritance is Not Supported in Java
- Why Java is not a pure Object Oriented language?
- Why can't a Java class be declared as static?
- Difference between Abstract class and Interface in Java
- Why do I need to override the equals and hashCode methods in Java?
- Why does Java not support operator overloading?
- What’s meant by anonymous class in Java?
- Static Vs Dynamic class loading in Java
- Why am I getting a NoClassDefFoundError in Java?
- How to generate random integers within a specific range in Java
- What's the meaning of System.out.println in Java?
- What is the purpose of Runtime and System class?
- What is finally block in Java?
- What is difference between final, finally and finalize?
- What is try-with-resources in java?
- What is a stacktrace?
- What is the meaning of immutable in terms of String?
- What are different ways to create a string object in Java?
- Difference between String and StringBuffer/StringBuilder in Java
- What is the difference between creating String as new() and literal?
- How do I convert String to Date object in Java?
- How do I create a Java string from the contents of a file?
- What actually causes a StackOverflow error in Java?
- Why is char preferred over String for storage of password in Java
- What is I/O Filter and how do I use it in Java?
- Serialization and Deserialization in Java
- Understanding transient variables in Java
- What is Externalizable in Java?
- What is the purpose of serialization/deserialization in Java?
- What is the Difference between byte stream and Character streams
- How to append text to an existing file in Java
- Read/convert an InputStream to a String in Java
- What is the difference between Reader and InputStream in Java
- Introduction to Java threads
- What is synchronization Java?
- Static synchronization Vs non static synchronization in Java
- Java Thread Deadlock Tutorial
- What is Daemon thread in Java
- Difference between implements Runnable and extends Thread in Java
- What is the volatile keyword in Java
- What are the basic interfaces of Java Collections Framework
- What are the differences between ArrayList and Vector in Java
- What is the difference between ArrayList and LinkedList?
- What is the difference between List and Set in Java
- Difference between HashSet and HashMap in Java
- Difference between HashMap and Hashtable in Java?
- How does the hashCode() method of java works?
- Difference between capacity() and size() of Vector in Java