Java class initialization timing

One o'clock eye

1 the following six kinds of ways, the system will initialize the class or interface.

• Create an instance of the class. Create an instance of a class includes the use of the new operator to create an instance to create an instance by reflection, to create an instance by way deserialized.

• Call the static method of a class.

• Access to a class or static properties of the interface, or assignment for the static properties.

• To force the creation of a class or an interface corresponding to the object using java.lang.Class reflection mode. For example the code: Class.forName ( "Person").

• Subclass initialization of a class, subclass initialized when a class, all the parent of the child class will be initialized.

• Direct use java.exe command to run a master class, when you run a master class, the program will first initialize the main class.

2 final type static property, if the property can get property values ​​at compile time can be considered as the property may be compile-time constant. When a program uses compile-time constants, the system will think this is a passive use of the class, so it will not lead to initialize the class.

Two combat

Code 1

class MyTest
{
   static
   {
      System.out.println("静态初始化块...");
   }
   // 使用一个字符串直接量为static final的类变量赋值
   static final String compileConstant = "疯狂Java讲义";
}
public class CompileConstantTest
{
   public static void main(String[] args)
   {
      // 访问、输出MyTest中的compileConstant类变量
      System.out.println(MyTest.compileConstant);   // ①
   }
}

2 runs

Crazy Java handouts

3 Description

Using a modified static final class, and its value can be determined at compile time down, then do not initialize the class.

Three combat

Code 1

class Tester
{
   static
   {
      System.out.println("Tester类的静态初始化块...");
   }
}
public class ClassLoaderTest
{
   public static void main(String[] args)
         throws ClassNotFoundException
   {
      ClassLoader cl = ClassLoader.getSystemClassLoader();
      // 下面语句仅仅是加载Tester类
      cl.loadClass("Tester");
      System.out.println("系统加载Tester类");
      // 下面语句才会初始化Tester类
      Class.forName("Tester");
   }
}

2 runs

系统加载Tester类
Tester类的静态初始化块...

3 Description

loadClass loads only the class, and it does not perform the initialization of the class.

forName lead to arbitrary initialization class.