In what order do static/instance initializer blocks in Java run?

问题

Say a project contains several classes, each of which has a static initializer block. In what order do those blocks run? I know that within a class, such blocks are run in the order they appear in the code. I\'ve read that it\'s the same across classes, but some sample code I wrote disagrees with that. I used this code:

package pkg;

public class LoadTest {
    public static void main(String[] args) {
        System.out.println(\"START\");
        new Child();
        System.out.println(\"END\");
    }
}

class Parent extends Grandparent {
    // Instance init block
    {
        System.out.println(\"instance - parent\");
    }

    // Constructor
    public Parent() {
        System.out.println(\"constructor - parent\");
    }

    // Static init block
    static {
        System.out.println(\"static - parent\");
    }
}

class Grandparent {
    // Static init block
    static {
        System.out.println(\"static - grandparent\");
    }

    // Instance init block
    {
        System.out.println(\"instance - grandparent\");
    }

    // Constructor
    public Grandparent() {
        System.out.println(\"constructor - grandparent\");
    }
}

class Child extends Parent {
    // Constructor
    public Child() {
        System.out.println(\"constructor - child\");
    }

    // Static init block
    static {
        System.out.println(\"static - child\");
    }

    // Instance init block
    {
        System.out.println(\"instance - child\");
    }
}

and got this output:

START
static - grandparent
static - parent
static - child
instance - grandparent
constructor - grandparent
instance - parent
constructor - parent
instance - child
constructor - child
END

The obvious answer from that is that parents\' blocks run before their children\'s, but that could just be a coincidence and doesn\'t help if two classes aren\'t in the same hierarchy.

EDIT:

I modified my example code by appending this to LoadTest.java:

class IAmAClassThatIsNeverUsed {
    // Constructor
    public IAmAClassThatIsNeverUsed() {
        System.out.println(\"constructor - IAACTINU\");
    }

    // Instance init block
    {
        System.out.println(\"instance - IAACTINU\");
    }

    // Static init block
    static {
        System.out.println(\"static - IAACTINU\");
    }
}

As implied by the class name, I never referenced the new class anywhere. The new program produced the same output as the old one.

回答1:

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.

So, for multiple classes, this totally depends on the code that's run to cause those classes to get loaded.

回答2:

See section 12.4 and 12.5 of the JLS version 8, they go into gory detail about all of this (12.4 for static and 12.5 for instance variables).

For static initialization (section 12.4):

A class or interface type T will be initialized immediately before the first occurrence of any one of the following:

T is a class and an instance of T is created.
T is a class and a static method declared by T is invoked.
A static field declared by T is assigned.
A static field declared by T is used and the field is not a constant variable (§4.12.4).
T is a top level class (§7.6), and an assert statement (§14.10) lexically nested within T (§8.1.3) is executed.

(and several weasel-word clauses)

回答3:

Keith's and Chris's answers are both great, I'm just adding some more detail for my specific question.

Static init blocks run in the order in which their classes are initialized. So, what order is that? Per JLS 12.4.1:

A class or interface type T will be initialized immediately before the first occurrence of any one of the following:

T is a class and an instance of T is created.

T is a class and a static method declared by T is invoked.

A static field declared by T is assigned.

A static field declared by T is used and the field is not a constant variable (§4.12.4).

T is a top-level class, and an assert statement (§14.10) lexically nested within T is executed.

Invocation of certain reflective methods in class Class and in package java.lang.reflect also causes class or interface initialization. A class or interface will not be initialized under any other circumstance.

To illustrate, here's a walkthrough of what's happening in the example:

Enter main
Print "START"
Attempt to create first instance of Child, which requires initialization of Child
Attempting to initialize Child causes initialization of Parent
Attempting to initialize Parent causes initialization of Grandparent
At the start of initialization of Grandparent, Grandparent's static initialization block is run
Technically, Object gets the last say in the initialization chain by virtue of being Grandparent's parent, but it has nothing to contribute
After Grandparent's static initialization block ends, program falls back to Parent's static initialization block
After Parent's static initialization block ends, program falls back to Child's static initialization block
At this point, Child is initialized, so its constructor may proceed
Since IAmAClassThatIsNeverUsed never gets referenced, none of its code ever runs, including static initializer blocks
The rest of this walkthrough doesn't concern static initializers and is included only for completeness
Child's constructor implicitly calls super() (i.e., Parent's constructor)
Parent's constructor implicitly calls super() (i.e., Grandparent's constructor)
Grandparent's constructor does the same, which has no effect (again, Object has nothing to contribute)
Immediately after Grandparent's constructor's call to super() comes Grandparent's instance initializer block
The rest of Grandparent's constructor's constructor runs and the constructor terminates
The program falls back to Parent's constructor, immediately after its call to super() (i.e., Grandparent's constructor) resolves
As above, Parent's instance initializer does its thing and its constructor finishes up
Similarly, the program returns to and completes Child's constructor
At this point, the object has been instantiated
Print "END"
Terminate normally

回答4:

Initialization of a class consists of executing its static initializers and the initializers for static fields (class variables) declared in the class.

Initialization of an interface consists of executing the initializers for fields (constants) declared in the interface.

Before a class is initialized, its direct superclass must be initialized, but interfaces implemented by the class are not initialized. Similarly, the superinterfaces of an interface are not initialized before the interface is initialized.

回答5:

You can have multiple static and instance initializers in the same class, therefore

Static initializers are called in the textual order they are declared (from 12.4.2)
Instance initializers are called in the textual order they are declared (from 12.5)

Each is executed as if it was a single block.

回答6:

http://docs.oracle.com/javase/tutorial/java/javaOO/initial.html

kindly check java documentation.

then clearly mentioned no matter how may static blocks are there they will be executed as a single block in the order they appear

So,

My understanding here is java is looking your code as

static{
i=1;
i=2;
}

static int i;

that is why you are getting output 2

hope this is helpful

回答7:

There is one case in which a static block will not be called.

class Super {
    public static int i=10;
}
class Sub extends Super {
    static {
        system.out.println("Static block called");
    }
}
class Test {
    public static void main (String [] args) {
        system.out.println(Sub.i);
    } 
}

The above code outputs 10

回答8:

class A {
  public A() { 
    // 2
  }
}

class B extends A{
  static char x = 'x'; // 0
  char y = 'y'; // 3
  public B() { 
    // 4
  }

  public static void main(String[] args) {
    new B(); // 1
  }
}

Numbers in the comment indicate the evaluation order, the smaller, the earlier.

As the example showed,

static variable
main
constructor of superclass
instance variable
constructor

来源：https://stackoverflow.com/questions/2007666/in-what-order-do-static-instance-initializer-blocks-in-java-run

标签

java

static-initializer