咱們都知道JAVA初始化一個類的時候java
1.能夠用new 操做符來初始化,這是動態加載數據庫
2.也可經過Class.forName的方式來獲得一個Class類型的實例,而後經過這個Class類型的實例的newInstance來初始化.這是動態加載.在不少框架中常常使用,經過用property文件的方式指定類名稱.如咱們熟悉的在寫JDBC代碼的時候加載不一樣數據庫的驅動.框架
靜態加載的時候若是在運行環境中找不到要初始化的類,拋出的是NoClassDefFoundError,它在JAVA的異常體系中是一個Error,錯誤.this
動態態加載的時候若是在運行環境中找不到要初始化的類,拋出的是ClassNotFoundException,它在JAVA的異常體系中是一個checked異常,在寫代碼的時候就須要catch..net
下面用實例來演示:ReferencedCls是須要初始化的類,它在一個單獨的jar中,它被StaticReferencingCls和DynamicReferencingCls分別以靜態和動態的方式加載.blog
將prjT1作成一個jar包,t1.jar.將prjT2作成一個jar包,t2.jar.放在d:/temp文件夾.ci
JAVA代碼以下:
package com.test1;
public class ReferencedCls {
private String str = "test value";
public String getStr() {
return str;
}
public void setStr(String str) {
this.str = str;
}
public ReferencedCls() {
System.out.println("Constructor: ReferencedCls");
}
}
package com.test2;
import com.test1.ReferencedCls;
public class StaticReferencingCls {
public static void main(String[] args) {
System.out.println("Started testing static loading...............");
ReferencedCls cls = new ReferencedCls();
System.out.println(cls.getStr());
}
}
package com.test2;
import com.test1.ReferencedCls;
public class DynamicReferencingCls {
public static void main(String[] args) throws ClassNotFoundException, InstantiationException, IllegalAccessException {
System.out.println("Started testing dynamic loading...............");
Class cls = Class.forName("com.test1.ReferencedCls");
ReferencedCls obj = (ReferencedCls)cls.newInstance();
System.out.println(obj.getStr());
}
}
以不一樣的方式運行:
1,在運行環境中能找到須要的類時,都正常運行
D:\>java -cp d:/temp/t1.jar;d:/temp/t2.jar com.test2.StaticReferencingCls
Started testing static loading...............
Constructor: ReferencedCls
test value
D:\>java -cp d:/temp/t1.jar;d:/temp/t2.jar com.test2.DynamicReferencingCls
Started testing dynamic loading...............
Constructor: ReferencedCls
test value
2,以靜態方式加載,運行環境缺失須要的類,拋出NoClassDefFoundError
D:\>java -cp d:/temp/t2.jar com.test2.StaticReferencingCls
Started testing static loading...............
Exception in thread "main" java.lang.NoClassDefFoundError: com/test1/ReferencedCls
at com.test2.StaticReferencingCls.main(StaticReferencingCls.java:9)
Caused by: java.lang.ClassNotFoundException: com.test1.ReferencedCls
at java.net.URLClassLoader$1.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClassInternal(Unknown Source)
... 1 more
3,以動態方式加載,運行環境缺失須要的類,拋出ClassNotFoundException,同時在寫代碼的時候也須要catch這個異常或者聲明這個異常.
D:\>java -cp d:/temp/t2.jar com.test2.DynamicReferencingCls
Started testing dynamic loading...............
Exception in thread "main" java.lang.ClassNotFoundException: com.test1.ReferencedCls
at java.net.URLClassLoader$1.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClassInternal(Unknown Source)
at java.lang.Class.forName0(Native Method)
at java.lang.Class.forName(Unknown Source)
at com.test2.DynamicReferencingCls.main(DynamicReferencingCls.java:11)
4,JVM自己使用的方式也是靜態加載,如用JAVA命令運行一個帶main方法的類,這個類在運行環境缺失.
D:\>java com.test2.XXXCls
Exception in thread "main" java.lang.NoClassDefFoundError: com/test2/XXXCls
Caused by: java.lang.ClassNotFoundException: com.test2.XXXCls
at java.net.URLClassLoader$1.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClassInternal(Unknown Source)
Could not find the main class: com.test2.XXXCls. Program will exit.
虛擬機會延遲加載類,不是import的類都會被加載,只有在第一次使用的時候才加載。
好比下面的代碼,若是不知足條件,XClass可能在整個運行期間都不會被加載。
if("1".equals(args[0])){
System.out.println(XClass.staticStr);
}get
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做者:IT農夫
來源:CSDN
原文:https://blog.csdn.net/kkdelta/article/details/7637562
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