<init>與<clinit>的區別
今天在看深入Java虛擬機的class文件結構時,看到了這麼一句話,
可能出現在class文件中的兩種編譯器產生的方法是:實例初始化方法(名爲<init>)和類與接口初始化方法(名爲<clinit>)。
這兩種方法有什麼區別呢?
首先:這兩個方法一個是虛擬機在裝載一個類初始化的時候調用的(clinit)。另一個是在類實例化時調用的(init)
首先說說類的初始化:
在Java代碼中,一個正確的初始值是通過類變量初始化語句或者靜態初始化語句給出的。一個類變量初始化語句是
變量聲明後的等號和表達式:
- class Example {
- static int size = 3 * (int) (Math.random() * 5.0);
- }
class Example {
static int size = 3 * (int) (Math.random() * 5.0);
}
靜態初始化語句是一個以static開頭的語句塊:
- class Example{
- static int size;
- static {
- size = 3 * (int) (Math.random() * 5.0);
- }
- }
class Example{
static int size;
static {
size = 3 * (int) (Math.random() * 5.0);
}
}
所有的類變量初始化語句和類型的靜態初始化語句都被Java編譯器收集到了一起,放在一個特殊的方法中。這個方法就是<clinit>
我們在來看看<init>這個方法:
<init>方法是在一個類進行對象實例化時調用的。實例化一個類有四種途徑:調用new操作符;調用Class或java.lang.reflect.Constructor對象的newInstance()方法;調用任何現有對象的clone()方法;通過java.io.ObjectInputStream類的getObject()方法反序列化。
Java編譯器會爲它的每一個類都至少生成一個實例初始化方法。在Class文件中,被稱爲"<init>"
現在知道了吧, 一個是用於初始化靜態的類變量, 一個是初始化實例變量!
在編譯生成class文件時,會自動產生兩個方法,一個是類的初始化方法<clinit>, 另一個是實例的初始化方法<init>
<clinit>:在jvm第一次加載class文件時調用,包括靜態變量初始化語句和靜態塊的執行
<init>:在實例創建出來的時候調用,包括調用new操作符;調用Class或java.lang.reflect.Constructor對象的newInstance()方法;調用任何現有對象的clone()方法;通過java.io.ObjectInputStream類的getObject()方法反序列化。
- import java.util.*;
- class ParentTest {
- static int y =2;
- int yy = 3;
- static {
- System.out.println("parentTest y = " + y);
- }
- {
- ++y;
- }
- ParentTest() {
- System.out.println("ParentTest construction y = " + y);
- }
- }
- public class Testextends ParentTest{
- /**
- * @param args
- */
- static int x =1;
- static String s = "123";
- static {
- if (s.equals("123"))
- s = "345";
- if (x == 1) {
- x = 2;
- }
- }
- {
- System.out.println("<init>");
- if (s.equals("345"))
- s = "678";
- if (x == 2)
- x = 3;
- ++x;
- }
- public Test() {
- System.out.println(x);
- System.out.println(s);
- }
- public Test(String ss) {
- System.out.println(x);
- System.out.println(s);
- }
- public staticvoid main(String[] args) {
- // TODO Auto-generated method stub
- new Test();
- System.out.println();
- new Test("sssss");
- //Test t = new Test("333");
- //System.out.println(t.x);
- //System.out.println(Test.s);
- }
- }
import java.util.*;
class ParentTest {
static int y = 2;
int yy = 3;
static {
System.out.println("parentTest y = " + y);
}
{
++y;
}
ParentTest() {
System.out.println("ParentTest construction y = " + y);
}
}
public class Test extends ParentTest{
/**
* @param args
*/
static int x = 1;
static String s = "123";
static {
if (s.equals("123"))
s = "345";
if (x == 1) {
x = 2;
}
}
{
System.out.println("<init>");
if (s.equals("345"))
s = "678";
if (x == 2)
x = 3;
++x;
}
public Test() {
System.out.println(x);
System.out.println(s);
}
public Test(String ss) {
System.out.println(x);
System.out.println(s);
}
public static void main(String[] args) {
// TODO Auto-generated method stub
new Test();
System.out.println();
new Test("sssss");
//Test t = new Test("333");
//System.out.println(t.x);
//System.out.println(Test.s);
}
}
output:
parentTest y = 2
ParentTest construction y = 3
<init>
4
678
ParentTest construction y = 4
<init>
5
678
.class public Lcom/a/b/c/a;
.super Ljava/lang/Object;
# static fields
.field public static final e:Landroid/net/Uri;
.field public static final f:Landroid/net/Uri;
.field private static final m:Landroid/net/Uri;
.field private static final n:Landroid/net/Uri;
# instance fields
.field a:Ljava/lang/String;
.field b:Ljava/lang/String;
.field c:I
.field d:I
.field g:Landroid/net/ConnectivityManager;
.field h:Landroid/net/NetworkInfo;
.field i:Lcom/a/b/c/b;
.field private j:Landroid/content/Context;
.field private k:I
.field private l:I
.field private o:Ljava/lang/String;
# direct methods
.method static constructor <clinit>()V
.locals 1
const-string v0, "content://telephony/carriers"
invoke-static {v0}, Landroid/net/Uri;->parse(Ljava/lang/String;)Landroid/net/Uri;
move-result-object v0
sput-object v0, Lcom/a/b/c/a;->m:Landroid/net/Uri;
const-string v0, "content://telephony/carriers/preferapn"
invoke-static {v0}, Landroid/net/Uri;->parse(Ljava/lang/String;)Landroid/net/Uri;
move-result-object v0
sput-object v0, Lcom/a/b/c/a;->n:Landroid/net/Uri;
const-string v0, "content://telephony/carriers/preferapn"
invoke-static {v0}, Landroid/net/Uri;->parse(Ljava/lang/String;)Landroid/net/Uri;
move-result-object v0
sput-object v0, Lcom/a/b/c/a;->e:Landroid/net/Uri;
const-string v0, "content://telephony/carriers"
invoke-static {v0}, Landroid/net/Uri;->parse(Ljava/lang/String;)Landroid/net/Uri;
move-result-object v0
sput-object v0, Lcom/a/b/c/a;->f:Landroid/net/Uri;
return-void
.end method
.method public constructor <init>(Landroid/content/Context;)V
.locals 2
const/4 v1, -0x1
invoke-direct {p0}, Ljava/lang/Object;-><init>()V
const/4 v0, 0x0
iput v0, p0, Lcom/a/b/c/a;->k:I
iput v1, p0, Lcom/a/b/c/a;->c:I
iput v1, p0, Lcom/a/b/c/a;->d:I
iput v1, p0, Lcom/a/b/c/a;->l:I
iput-object p1, p0, Lcom/a/b/c/a;->j:Landroid/content/Context;
invoke-direct {p0}, Lcom/a/b/c/a;->g()Ljava/lang/String;
move-result-object v0
iput-object v0, p0, Lcom/a/b/c/a;->a:Ljava/lang/String;
invoke-direct {p0}, Lcom/a/b/c/a;->h()Ljava/lang/String;
move-result-object v0
iput-object v0, p0, Lcom/a/b/c/a;->b:Ljava/lang/String;
const/4 v0, 0x1
invoke-direct {p0, v0}, Lcom/a/b/c/a;->a(Z)I
return-void
.end method
package com.a.b.c;
import android.content.*;
import android.database.Cursor;
import android.database.SQLException;
import android.net.*;
import android.telephony.TelephonyManager;
import android.util.Log;
import com.ac.f;
import java.lang.reflect.Method;
// Referenced classes of package com.a.b.c:
// b, c
public class a
{
public static final Uri e = Uri.parse("content://telephony/carriers/preferapn");
public static final Uri f = Uri.parse("content://telephony/carriers");
private static final Uri m = Uri.parse("content://telephony/carriers");
private static final Uri n = Uri.parse("content://telephony/carriers/preferapn");
String a;
String b;
int c;
int d;
ConnectivityManager g;
NetworkInfo h;
b i;
private Context j;
private int k;
private int l;
private String o;
public a(Context context)
{
k = 0;
c = -1;
d = -1;
l = -1;
j = context;
a = g();
b = h();
a(true);
}