public class A {
public static String HOST;
static {
HOST = ...;
}
}
public class B {
public static String URL;
static{
URL = A.HOST + ...;
}
}
My question is if A.HOST would be correctly initialised before B use it? Is this behaviour is defined in spec?
Yes that behavior is well defined here.
In short, citing from that link
Initialization of a class or interface consists of executing the class or interface initialization method
<clinit>
...
A class or interface may be initialized only as a result of:
The execution of any one of the Java Virtual Machine instructions new, getstatic, putstatic, or invokestatic that references the class or interface (§new, §getstatic, §putstatic, §invokestatic). All of these instructions reference a class directly or indirectly through either a field reference or a method reference.
Upon execution of a new instruction, the referenced class or interface is initialized if it has not been initialized already.
Upon execution of a getstatic, putstatic, or invokestatic instruction, the class or interface that declared the resolved field or method is initialized if it has not been initialized already.
The first invocation of a java.lang.invoke.MethodHandle instance which was the result of resolution of a method handle by the Java Virtual Machine (§5.4.3.5) and which has a kind of 2 (REF_getStatic), 4 (REF_putStatic), or 6 (REF_invokeStatic).
Invocation of certain reflective methods in the class library (§2.12), for example, in class Class or in package java.lang.reflect.
The initialization of one of its subclasses.
Its designation as the initial class at Java Virtual Machine start-up (§5.2).
The <clinit> method is the method (created by the compiler) that initializes static variables and has the code that you put in the static block
In your case, when the static block of class B runs (which is what <clinit> will do), it will have a getStatic opcode, requesting A.HOST. So the initialization of A will be triggered, and A.HOST initialized. So you will read the proper value.
The static block for a class gets executed when the
class is accessed, either to create an instance, or to access a static method or field.
This depends on the code that we are executing.
In your case, when we do A.HOST, it calls static block of class A also.
Refer this
The static initializer for a class gets run when the class is first accessed, either to create an instance, or to access a static method or field.
Source: In what order do static initializer blocks in Java run?
I came across your post since I have similar doubt.
When there is a cycle dependency, it does looks like it is problematic in some cases. Here is my test code.
The output from https://www.tutorialspoint.com/online_java_compiler.php for below is ABC_null_TT2_EFG
class A {
public static String HOST;
static {
HOST = "ABC_" + B.TT + "_" + B.TT2;
}
}
public class B {
public static String URL;
static {
URL = A.HOST + "_EFG";
}
public static void main(String[] args) {
System.out.println(URL);
}
public static String TT = "TT";
public final static String TT2 = "TT2";
}
TT to before the initialiser block.A static block will be executed,before the main() method executes.
B.URL and A.HOST are both static. But B.URL initialisation requires A.HOST. So the problem is as A has not yet been initialised when B.URL is being initialised, will it cause any problem or not?
HOST = B.FOO. Not sure that one would even compile, though.