When are java classes loaded?

When is the class loaded?

The loading of the class is completed through the class loader (Classloader), which can be either Chinese-style [eagerly load] (loading as long as other classes reference it) loads a class, or it can be lazy load [lazy load] (wait until class initialization occurs before loading). (Recommended learning: java course)

But I believe this is related to different JVM implementations, but it is guaranteed by JLS (only when there is a static initialization requirement be loaded).

When is the class initialized?

After the class is loaded, the initialization of the class will occur, which means that it will initialize all class static members. A class is initialized in the following situations :

Instances are created by using the new() keyword or using class.forName() reflection, but it may cause ClassNotFoundException.

The static method of the class is called

The static field of the class is assigned

The static field is accessed, and it is not a constant

Executed in the top-level class assert statement

Reflection can also initialize a class, such as certain methods under the java.lang.reflect package. JLS strictly states: a class will not be initialized for any reason other than the above.

How a class is initialized

Now we know when the initialization of a class is triggered, it is precisely written in the Java Language Specification. But understand clearly the fields (fields, static or non-static), blocks (static or non-static), different classes (subclasses and superclasses) and different interfaces (subinterfaces, implementation classes and superinterfaces) The order of initialization is also important for classes.

In fact many core Java interview questions and SCJP questions are based on these concepts. Here are some rules for class initialization:

Class initialization in order from top to bottom , so fields declared at the top are initialized earlier than fields at the bottom

Super classes are initialized earlier than subclasses and derived classes

If the initialization of a class is triggered by accessing a static field, then Only the class that declares the static field is initialized, and does not trigger the initialization of the superclass or the initialization of the subclass even if the static field is referenced by a subclass or subinterface or its implementation class.

Interface initialization will not cause the initialization of the parent interface.

The initialization of static fields is during the static initialization of the class, and the initialization of non-static fields is during the creation of instances of the class. This means that static fields are initialized before non-static fields.

Non-static fields are initialized through the constructor. The constructor of the subclass will implicitly call the constructor of the parent class before doing any initialization. It ensures that non-static or instance variables (parent class) are initialized earlier than the child. kind

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