獲取ClassLoader的途徑
- 獲取當前類的ClassLoader
clazz.getCLassLoader(); - 獲取當前線程上下文的ClassLoader
Thread.currentThread().getContextClassLoader() - 獲得系統(tǒng)的ClassLoader
ClassLoader.getSystemClassLoader() - 獲得調用者的ClassLoader
DirverManager.getCallerClassLoader()
/**
* @author NingXioaoming
* @createTime 2019/12/16 13:56
* @description
*/
public class MyTest14 {
public static void main(String[] args) {
ClassLoader contextClassLoader = Thread.currentThread().getContextClassLoader();
System.out.println(contextClassLoader);
final Class<MyTest14> clazz = MyTest14.class;
System.out.println(clazz.getClassLoader());//由系統(tǒng)類加載sun.misc.Launcher$AppClassLoader@18b4aac2
final Class clazz1 = String.class;
System.out.println(clazz1.getClassLoader()); //由根類或者啟動類加載的包蓝，null 因為String位于rt.jar包中
}
}
數(shù)組對是在程序運行期間由jvm創(chuàng)建的而不是由類加載器加載的丈冬，調用數(shù)組的getClassLoader得到的結果是數(shù)組中對象所返回的結果碍拆，如果數(shù)組中的元素是原生類型（int性芬，double）的話調用getClassLoader是沒有類加載器的
自定義類加載器（[loader1]）和系統(tǒng)類加載器（sun.misc.Launcher$AppClassLoader@18b4aac2）的區(qū)別
package com.mobius.vision.jvm.classloader;
import java.io.ByteArrayOutputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.InputStream;
/**
* @author NingXioaoming
* @createTime 2019/12/21 10:43
* @description
*/
public class MyTest16 extends ClassLoader {
private String classLoaderName;
private final String fileExtension = ".class";
private String path;
public void setPath(String path) {
this.path = path;
}
public MyTest16(String classLoaderName){
super(); //將系統(tǒng)加載器當做該類加載器的父加載器
this.classLoaderName = classLoaderName;
}
public MyTest16(ClassLoader parent,String classLoaderName){
super(parent);//顯示指定該類加載器的父加載器
this.classLoaderName = classLoaderName;
// System.out.println("nihap111");
}
@Override
public String toString() {
return "["+this.classLoaderName+"]";
}
@Override
protected Class<?> findClass(String className) throws ClassNotFoundException {
byte[] data = this.loadClassData(className);
System.out.println("findClass invoked:" + className);
System.out.println("class loader name:"+ this.classLoaderName);
return this.defineClass(className,data,0,data.length);
}
private byte[] loadClassData(String className){
InputStream is = null;
byte[] data = null;
ByteArrayOutputStream baos = null;
className = className.replace(".","\\");
try {
is = new FileInputStream(new File(this.path+className+this.fileExtension));
baos = new ByteArrayOutputStream();
int ch;
while (-1 !=(ch = is.read())){
baos.write(ch);
}
data = baos.toByteArray();
}catch (Exception e){
e.printStackTrace();
}finally {
try {
is.close();
baos.close();
}catch (Exception e){
e.printStackTrace();
}
}
return data;
}
/* public static void test(ClassLoader classLoader) throws Exception{
final Class<?> clazz = classLoader.loadClass("com.mobius.vision.jvm.classloader.MyTest1");
final Object object = clazz.newInstance();
System.out.println(object);
// System.out.println(object.getClass().getClassLoader());
// System.out.println(clazz.getClassLoader());
}*/
public static void main(String[] args) throws Exception {
final MyTest16 loader1 = new MyTest16("loader1");
loader1.setPath("C:\\Users\\user\\Desktop\\");
final Class<?> clazz = loader1.loadClass("com.mobius.vision.jvm.classloader.MyTest1");
System.out.println("class:"+clazz.hashCode());
final Object object = clazz.newInstance();
System.out.println(object.getClass().getClassLoader());
// test(loader1);
System.out.println("************************");
final MyTest16 loader2 = new MyTest16("loader2");
loader2.setPath("C:\\Users\\user\\Desktop\\");
final Class<?> clazz2 = loader2.loadClass("com.mobius.vision.jvm.classloader.MyTest1");
System.out.println("class:"+clazz2.hashCode());
final Object object2 = clazz2.newInstance();
System.out.println(object2.getClass().getClassLoader());
}
}
自定義類加載器輸出的結果
- hashCode值是不同的 說明加載了兩次 類被實例化了兩次。
findClass invoked:com.mobius.vision.jvm.classloader.MyTest1
class loader name:loader1
class:325040804
[loader1]
************************
findClass invoked:com.mobius.vision.jvm.classloader.MyTest1
class loader name:loader2
class:621009875
[loader2]
Process finished with exit code 0
系統(tǒng)類加載器的輸出結果
- 兩次輸出相同的hashCode值 說明兩次第二次new出來的實例還是上一個實例化出來的對象
- 出現(xiàn)這個原因是因為 在加載類的時候 調用了loadClass方法瘦黑，這個方法首先就是執(zhí)行findLoaderClass(String s)：這個方法就是尋找已經(jīng)被加載的類來檢查這個類是否已經(jīng)被加載過了京革。
class:2133927002
sun.misc.Launcher$AppClassLoader@18b4aac2
************************
class:2133927002
sun.misc.Launcher$AppClassLoader@18b4aac2
Process finished with exit code 0
將代碼中第二次new的時候改成MyTest16 loader2 = new MyTest16(loader1,"loader2");變一個構造方法 將loader1的類加載器作為loader2的父加載器進行加載的輸出結果
- hashCode值相同
- 由于類加載的機制是雙親委托機制，所以
findClass invoked:com.mobius.vision.jvm.classloader.MyTest1
class loader name:loader1
class:325040804
[loader1]
************************
class:325040804
[loader1]
