Java Object Initialization Order
See the following program, and you guess the running result:
public class Dog {
public static Dog dog = new Dog();
static final int val1 = -5;
static int val2 = 3;
public int val3;
public Dog() {
val3 = val1 + val2;
}
public static void main(String[] args) {
System.out.println(Dog.dog.val3);
}
}
Now, I tell you the result is -5. But why?
To clarify this confusing initialization order, you must know a little about the memory layout and what the JVM do (imagine yourself as the compiler and VM).
Because static fields belong to the Class which located in the method area of JVM. The class initialization occurs in the class loading process by the class loader. The process is more or less like the one of plain object initialization other than the member fields of classes are static and class initialization doesn’t invoke any user-defined constructor.
One thing I must mention is that -5 is just CONSTANT, so assigning it to val1 means it is hard-corded into class file at compile-time. Therefore, final class variables and fields of interfaces whose values are compile-time constants are initialized first. 具有编译器常量的final类变量首先初始化！！ http://docs.oracle.com/javase/sp…
To verify your thought, please see the decompiled bytecodes:
Classfile /playground/bin/playground/Dog.class
Last modified Aug 25, 2016; size 725 bytes
MD5 checksum 73b977e92851a8923319ee5a1032171a
Compiled from "Dog.java"
public class playground.Dog
SourceFile: "Dog.java"
minor version: 0
major version: 51
flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
#1 = Class #2 // playground/Dog
#2 = Utf8 playground/Dog
#3 = Class #4 // java/lang/Object
#4 = Utf8 java/lang/Object
#5 = Utf8 dog
#6 = Utf8 Lplayground/Dog;
#7 = Utf8 val1
#8 = Utf8 I
#9 = Utf8 ConstantValue
#10 = Integer -5
#11 = Utf8 val2
#12 = Utf8 val3
#13 = Utf8 <clinit>
#14 = Utf8 ()V
#15 = Utf8 Code
#16 = Methodref #1.#17 // playground/Dog."<init>":()V
#17 = NameAndType #18:#14 // "<init>":()V
#18 = Utf8 <init>
#19 = Fieldref #1.#20 // playground/Dog.dog:Lplayground/Dog;
#20 = NameAndType #5:#6 // dog:Lplayground/Dog;
#21 = Fieldref #1.#22 // playground/Dog.val2:I
#22 = NameAndType #11:#8 // val2:I
#23 = Utf8 LineNumberTable
#24 = Utf8 LocalVariableTable
#25 = Methodref #3.#17 // java/lang/Object."<init>":()V
#26 = Fieldref #1.#27 // playground/Dog.val3:I
#27 = NameAndType #12:#8 // val3:I
#28 = Utf8 this
#29 = Utf8 main
#30 = Utf8 ([Ljava/lang/String;)V
#31 = Fieldref #32.#34 // java/lang/System.out:Ljava/io/PrintStream;
#32 = Class #33 // java/lang/System
#33 = Utf8 java/lang/System
#34 = NameAndType #35:#36 // out:Ljava/io/PrintStream;
#35 = Utf8 out
#36 = Utf8 Ljava/io/PrintStream;
#37 = Methodref #38.#40 // java/io/PrintStream.println:(I)V
#38 = Class #39 // java/io/PrintStream
#39 = Utf8 java/io/PrintStream
#40 = NameAndType #41:#42 // println:(I)V
#41 = Utf8 println
#42 = Utf8 (I)V
#43 = Utf8 args
#44 = Utf8 [Ljava/lang/String;
#45 = Utf8 SourceFile
#46 = Utf8 Dog.java
{
public static playground.Dog dog;
flags: ACC_PUBLIC, ACC_STATIC
static final int val1;
flags: ACC_STATIC, ACC_FINAL
ConstantValue: int -5
static int val2;
flags: ACC_STATIC
public int val3;
flags: ACC_PUBLIC
static {};
flags: ACC_STATIC
Code:
stack=2, locals=0, args_size=0
0: new #1 // class playground/Dog
3: dup
4: invokespecial #16 // Method "<init>":()V
7: putstatic #19 // Field dog:Lplayground/Dog;
10: iconst_3
11: putstatic #21 // Field val2:I
14: return
LineNumberTable:
line 4: 0
line 6: 10
LocalVariableTable:
Start Length Slot Name Signature
public playground.Dog();
flags: ACC_PUBLIC
Code:
stack=3, locals=1, args_size=1
0: aload_0
1: invokespecial #25 // Method java/lang/Object."<init>":()V
4: aload_0
5: bipush -5
7: getstatic #21 // Field val2:I
10: iadd
11: putfield #26 // Field val3:I
14: return
LineNumberTable:
line 8: 0
line 9: 4
line 10: 14
LocalVariableTable:
Start Length Slot Name Signature
0 15 0 this Lplayground/Dog;
public static void main(java.lang.String[]);
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=2, locals=1, args_size=1
0: getstatic #31 // Field java/lang/System.out:Ljava/io/PrintStream;
3: getstatic #19 // Field dog:Lplayground/Dog;
6: getfield #26 // Field val3:I
9: invokevirtual #37 // Method java/io/PrintStream.println:(I)V
12: return
LineNumberTable:
line 12: 0
line 13: 12
LocalVariableTable:
Start Length Slot Name Signature
0 13 0 args [Ljava/lang/String;
}
Let’s illustrate the whole process using pictures:
The following is a complicated example:
public class Test {
private static int k = 1;
private static Test t1 = new Test("t1");
private static Test t2 = new Test("t2");
private static int n = 99;
private static int j = print("j");
private int i = print("i");
{
print("构造块");
}
static {
print("静态构造块");
}
public Test(String str) {
System.out.println((++k) + ":" + str +" j="+j+" n="+n);
++j;
++n;
}
public static int print(String str) {
System.out.println((++k) + ":" + str +" j="+j+" n="+n);
++n;
return ++j;
}
public static void main(String[] args) {
}
}
the result is:
2:i j=0 n=0 3:构造块 j=1 n=1 4:t1 j=2 n=2 5:i j=3 n=3 6:构造块 j=4 n=4 7:t2 j=5 n=5 8:j j=6 n=99 9:静态构造块 j=7 n=100