Shulou(Shulou.com)06/01 Report--

Today, I will talk to you about the implementation of int and Integer caching, which may not be well understood by many people. in order to make you understand better, the editor has summarized the following content for you. I hope you can get something from this article.

Int I believe we are all familiar with the original data types in Java, as well as long, short, float, double, char, byte, and boolean. The original data type is not an object.

Integer is the wrapper class for int, which has a private final int value; to store data and also provides some basic operations, such as sum, max, int, and string conversion. From the declaration of value as private final, we can see that it is also an immutable type!

Auto-boxing and auto-unboxing are introduced in Java5, that is, Java can automatically convert int to Integer or Integer to int according to the context. Automatic packing and unpacking is actually a kind of grammatical sugar (grammatical sugar is a package of existing syntax, mainly to facilitate programmers' development and improve development efficiency).

It happens during the compilation phase, and javac helps us to automatically box and unpack. Specifically, the Integer.valueOf method is called to box and the Integer.intValue method is called to unpack. Let's decompile and see if this is the case, such as the following code

The result of decompilation is indeed the case!

Let's talk about this valueOf method again. The traditional way to build an Integer object is to directly call the constructor new an object, but according to practice, it is found that most of the data operations are concentrated in a relatively small range of values, so the static factory method valueOf is introduced in Java5, which is called using IntegerCache, that is, a cache, which can bring significant performance improvement. The default cache value is-128 to 127.

What do you mean? Take a look at the source code first

If the default cache value is not modified, we execute Integer I = a value (note that it is not direct new Integer, direct new will not make use of the cache). If this value is greater than or equal to-128and less than or equal to 127, it will not new a new Integer object but will go to IntegerCache to find a reference to the cache object and return directly. In this way, some of our commonly used values can be cached directly! Reduced consumption!

If you clearly know that the commonly used integer value is larger, you can also change the default cache value.

Change the maximum value of the cache by setting JVM-XX:AutoBoxCacheMax=, but the minimum value cannot be changed.

Let's see what IntegerCache has done.

It is very simple, it is to make an Integer array, read it in the static code block to see if we have set-XX:AutoBoxCacheMax=, then initialize the cache array with the default cache value directly, and put each Integer into the array. If integerCacheHighPropValue is set, modify the maximum cache value, and then operate.

One more thing to note: avoid inadvertently boxing and unboxing, especially in performance-sensitive situations, because creating a large number of objects and a large number of integers is not an order of magnitude in terms of memory or processing speed. So try to use raw data types in performance-sensitive situations.

After reading the above, do you have any further understanding of the implementation of int and Integer caching? If you want to know more knowledge or related content, please follow the industry information channel, thank you for your support.

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