Why I can't create an array with large size?
java, jvm, jvm-hotspot
Solution
Theory
There are two possible exceptions:
- `OutOfMemoryError: Java heap space` means your array does not fit into java heap space. In order to solve you can increase the maximum heap size by using JVM option `-Xmx`. Also take into account that the maximum size of object cannot be larger than the largest heap generation.
- `OutOfMemoryError: Requested array size exceeds VM limit` means platform-specific size was exceeded:
- the upper bound limit is set by the restrictions of the size type used to describe an index in the array, so theoretical array size is limited by `2^31-1=2147483647` elements.
- the other limit is JVM/platform specific. According to chapter 10: Arrays of The Java Language Specification, Java SE 7 Edition there is no strict limit on array length, thus array size may be reduced without violating JLS.
Practice
In HotSpot JVM array size is limited by internal representation. In the GC code JVM passes around the size of an array in heap words as an `int` then converts back from heap words to `jint` this may cause an overflow. So in order to avoid crashes and unexpected behavior the maximum array length is limited by (max size - header size). Where header size depends on C/C++ compiler which was used to build the JVM you are running(gcc for linux, clang for macos), and runtime settings(like `UseCompressedClassPointers`). For example on my linux:
- Java HotSpot(TM) 64-Bit Server VM 1.6.0_45 limit `Integer.MAX_VALUE`
- Java HotSpot(TM) 64-Bit Server VM 1.7.0_72 limit `Integer.MAX_VALUE-1`
- Java HotSpot(TM) 64-Bit Server VM 1.8.0_40 limit `Integer.MAX_VALUE-2`
Useful Links
- https://bugs.openjdk.java.net/browse/JDK-8059914
- https://bugs.openjdk.java.net/browse/JDK-8029587
Problem
Why it is impossible to create an array with max int size? ``` int i = 2147483647; int[] array = new int[i]; ``` I found this explanation: Java arrays are accessed via 32-bit ints, resulting in a maximum theoretical array size of 2147483647 elements. But as you can see my code doesn't work. It is also impossible to create an array with size ``` new int[Integer.MAX_VALUE - 5]; ``` Technical details - 64-Bit HotSpot JVM - OSX 10.10.4 PS And why `-5` actually?