":()V
19: athrow
我们可以看到,类初始化正是按照源文件中定义的原文顺序进行。先是声明 static int x;
static java.lang.String s;
然后对int x和String s进行赋值：
0: iconst_0
1: putstatic #4; //Field x:I
4: ldc #5; //String 123
6: putstatic #6; //Field s:Ljava/lang/String;
执行初始化块的String s1 = "456";生成一个RuntimeException抛
9: ldc #7; //String 456
11: astore_0
12: new #8; //class java/lang/RuntimeException
15: dup
16: invokespecial #9; //Method java/lang/RuntimeException."":()V
19: athrow
要明白的是，"<clinit>"方法不仅是类初始化方法，而且也是接口初始化方法。并不是所以接口
的属性都是内联的，只有直接赋常量值的接口常量才会内联。而
[public static final] double d = Math.random()*100;
这样的表达式是需要计算的，在接口中就要由"<clinit>"方法来初始化。
下面我们再来看看实例初始化方法"<init>"
"<init>"用于对象创建时对对象进行初始化，当在HEAP中创建对象时，一旦在HEAP分配了空间。最先就会调用"<init>"方法。这个方法包括实例变量的赋值（声明不在其中）和初始化块，以及构造方法调用。如果有多个重载的构造方法，每个构造方法都会有一个对应的"<init>"方法。构造方法隐式或显示调用父类的构造方法前,总是先执行实例变量初始化和初始化块.同样，实例变量和初始化块的顺序也是按源文件的原文顺序执行，构造方法中的代码在最后执行：
package debug;
public class Test {
int x = 0;
String s = "123";
{
String s1 = "456";
//if(1==1)
//throw new RuntimeException();
}
public Test(){
String ss = "789";
}
public static void main(String[] args) {
new Test();
}
}
javap -c debug.Test的结果：
Compiled from "Test.java"
public class debug.Test extends java.lang.Object{
int x;
java.lang.String s;
public debug.Test();
Code:
0: aload_0
1: invokespecial #1; //Method java/lang/Object."":()V
4: aload_0
5: iconst_0
6: putfield #2; //Field x:I
9: aload_0
10: ldc #3; //String 123
12: putfield #4; //Field s:Ljava/lang/String;
15: ldc #5; //String 456
17: astore_1
18: ldc #6; //String 789
20: astore_1
21: return
public static void main(java.lang.String[]);
Code:
0: new #7; //class debug/Test
3: dup
4: invokespecial #8; //Method "":()V
7: pop
8: return
}
如果在同一个类中，一个构造方法调用了另一个构造方法，那么对应的"<init>"方法就会调用另一
个"<init>"，但是实例变量和初始化块会被忽略，否则它们就会被多次执行。
package debug;
public class Test {
String s1 = rt("s1");
String s2 = "s2";
public Test(){
s1 = "s1";
}
public Test(String s){
this();
if(1==1) throw new Runtime();
}
String rt(String s){
return s;
}
public static void main(String[] args) {
new Test("");
}
}
反编译的结果：
Compiled from "Test.java"
public class debug.Test extends java.lang.Object{
java.lang.String s1;
java.lang.String s2;
public debug.Test();
Code:
0: aload_0
1: invokespecial #1; //Method java/lang/Object."":()V
4: aload_0
5: aload_0
6: ldc #2; //String s1
8: invokevirtual #3; //Method rt:(Ljava/lang/String;)Ljava/lang/String;
11: putfield #4; //Field s1:Ljava/lang/String;
14: aload_0
15: ldc #5; //String s2
17: putfield #6; //Field s2:Ljava/lang/String;
20: aload_0
21: ldc #2; //String s1
23: putfield #4; //Field s1:Ljava/lang/String;
26: return
public debug.Test(java.lang.String);
Code:
0: aload_0
1: invokespecial #7; //Method "":()V
4: new #8; //class java/lang/RuntimeException
7: dup
8: invokespecial #9; //Method java/lang/RuntimeException."":()V
11: athrow
java.lang.String rt(java.lang.String);
Code:
0: aload_1
1: areturn
public static void main(java.lang.String[]);
Code:
0: new #10; //class debug/Test
3: dup
4: ldc #11; //String
6: invokespecial #12; //Method "":(Ljava/lang/String;)V
9: pop
10: return
}
我们看到，由于Test(String s)调用了Test();所以"<init>":(Ljava/lang/String;)V不再对
实例变量和初始化块进次初始化：
public debug.Test(java.lang.String);
Code:
0: aload_0
1: invokespecial #7; //Method "":()V
4: new #8; //class java/lang/RuntimeException
7: dup
8: invokespecial #9; //Method java/lang/RuntimeException."":()V
11: athrow
而如果两个构造方法是相互独立的，则每个构造方法调用前都会执行实例变量和初始化块的调用：
package debug;
public class Test {
String s1 = rt("s1");
String s2 = "s2";
{
String s3 = "s3";
}
public Test() {
s1 = "s1";
}
public Test(String s) {
if (1 == 1)
throw new RuntimeException();
}
String rt(String s) {
return s;
}
public static void main(String[] args) {
new Test("");
}
}
反编译的结果：
Compiled from "Test.java"
public class debug.Test extends java.lang.Object{
java.lang.String s1;
java.lang.String s2;
public debug.Test();
Code:
0: aload_0
1: invokespecial #1; //Method java/lang/Object."":()V
4: aload_0
5: aload_0
6: ldc #2; //String s1
8: invokevirtual #3; //Method rt:(Ljava/lang/String;)Ljava/lang/String;
11: putfield #4; //Field s1:Ljava/lang/String;
14: aload_0
15: ldc #5; //String s2
17: putfield #6; //Field s2:Ljava/lang/String;
20: ldc #7; //String s3
22: astore_1
23: aload_0
24: ldc #2; //String s1
26: putfield #4; //Field s1:Ljava/lang/String;
29: return
public debug.Test(java.lang.String);
Code:
0: aload_0
1: invokespecial #1; //Method java/lang/Object."":()V
4: aload_0
5: aload_0
6: ldc #2; //String s1
8: invokevirtual #3; //Method rt:(Ljava/lang/String;)Ljava/lang/String;
11: putfield #4; //Field s1:Ljava/lang/String;
14: aload_0
15: ldc #5; //String s2
17: putfield #6; //Field s2:Ljava/lang/String;
20: ldc #7; //String s3
22: astore_2
23: new #8; //class java/lang/RuntimeException
26: dup
27: invokespecial #9; //Method java/lang/RuntimeException."":()V
30: athrow
java.lang.String rt(java.lang.String);
Code:
0: aload_1
1: areturn
public static void main(java.lang.String[]);
Code:
0: new #10; //class debug/Test
3: dup
4: ldc #11; //String
6: invokespecial #12; //Method "":(Ljava/lang/String;)V
9: pop
10: return
}
明白了上面这些知识,我们来做一个小测试吧:
public class Test2 extends Test1{
{
System.out.print("1");
}
Test2(){
System.out.print("2");
}
static{
System.out.print("3");
}
{
System.out.print("4");
}
public static void main(String[] args) {
new Test2();
}
}
class Test1 {
Test1(){
System.out.print("5");
}
static{
System.out.print("6");
}
}
试试看能清楚打印的顺序吗?如果没有new Test2()将打印什么?
作者:axman 专栏:http://blog.csdn.net/axman/