類加載器負(fù)責(zé)的范圍,首先看張圖
通過(guò)代碼驗(yàn)證如下結(jié)論
public class MyTest18 {
public static void main(String[] args) {
System.out.println(System.getProperty("sun.boot.class.path"));
System.out.println(System.getProperty("java.ext.dirs"));
System.out.println(System.getProperty("java.class.path"));
}
}
在idea里面運(yùn)行上述程序,會(huì)的到如下結(jié)果
啟動(dòng)類加載器
/Library/Java/JavaVirtualMachines/jdk1.8.0_221.jdk/Contents/Home/jre/lib/resources.jar:
/Library/Java/JavaVirtualMachines/jdk1.8.0_221.jdk/Contents/Home/jre/lib/rt.jar:
/Library/Java/JavaVirtualMachines/jdk1.8.0_221.jdk/Contents/Home/jre/lib/sunrsasign.jar:
/Library/Java/JavaVirtualMachines/jdk1.8.0_221.jdk/Contents/Home/jre/lib/jsse.jar:
/Library/Java/JavaVirtualMachines/jdk1.8.0_221.jdk/Contents/Home/jre/lib/jce.jar:
/Library/Java/JavaVirtualMachines/jdk1.8.0_221.jdk/Contents/Home/jre/lib/charsets.jar:
/Library/Java/JavaVirtualMachines/jdk1.8.0_221.jdk/Contents/Home/jre/lib/jfr.jar:
/Library/Java/JavaVirtualMachines/jdk1.8.0_221.jdk/Contents/Home/jre/classes
擴(kuò)展類加載器
/Users/zj/Library/Java/Extensions:
/Library/Java/JavaVirtualMachines/jdk1.8.0_221.jdk/Contents/Home/jre/lib/ext:
/Library/Java/Extensions:
/Network/Library/Java/Extensions:
/System/Library/Java/Extensions:
/usr/lib/java
系統(tǒng)類加載器
為什么在當(dāng)前idea環(huán)境所編寫(xiě)的這些class會(huì)被自動(dòng)加載兄裂，因?yàn)闀?huì)idea會(huì)自動(dòng)將target/classes添加到classpath里面粉捻，因此會(huì)被系統(tǒng)類加載器所加載
/Library/Java/JavaVirtualMachines/jdk1.8.0_221.jdk/Contents/Home/jre/lib/charsets.jar:
/Library/Java/JavaVirtualMachines/jdk1.8.0_221.jdk/Contents/Home/jre/lib/deploy.jar:
/Library/Java/JavaVirtualMachines/jdk1.8.0_221.jdk/Contents/Home/jre/lib/ext/cldrdata.jar:
/Library/Java/JavaVirtualMachines/jdk1.8.0_221.jdk/Contents/Home/jre/lib/ext/dnsns.jar:
/Library/Java/JavaVirtualMachines/jdk1.8.0_221.jdk/Contents/Home/jre/lib/ext/jaccess.jar:
/Library/Java/JavaVirtualMachines/jdk1.8.0_221.jdk/Contents/Home/jre/lib/ext/jfxrt.jar:
/Library/Java/JavaVirtualMachines/jdk1.8.0_221.jdk/Contents/Home/jre/lib/ext/localedata.jar:
/Library/Java/JavaVirtualMachines/jdk1.8.0_221.jdk/Contents/Home/jre/lib/ext/nashorn.jar:
/Library/Java/JavaVirtualMachines/jdk1.8.0_221.jdk/Contents/Home/jre/lib/ext/sunec.jar:
/Library/Java/JavaVirtualMachines/jdk1.8.0_221.jdk/Contents/Home/jre/lib/ext/sunjce_provider.jar:
/Library/Java/JavaVirtualMachines/jdk1.8.0_221.jdk/Contents/Home/jre/lib/ext/sunpkcs11.jar:
/Library/Java/JavaVirtualMachines/jdk1.8.0_221.jdk/Contents/Home/jre/lib/ext/zipfs.jar:
/Library/Java/JavaVirtualMachines/jdk1.8.0_221.jdk/Contents/Home/jre/lib/javaws.jar:
/Library/Java/JavaVirtualMachines/jdk1.8.0_221.jdk/Contents/Home/jre/lib/jce.jar:
/Library/Java/JavaVirtualMachines/jdk1.8.0_221.jdk/Contents/Home/jre/lib/jfr.jar:
/Library/Java/JavaVirtualMachines/jdk1.8.0_221.jdk/Contents/Home/jre/lib/jfxswt.jar:
/Library/Java/JavaVirtualMachines/jdk1.8.0_221.jdk/Contents/Home/jre/lib/jsse.jar:
/Library/Java/JavaVirtualMachines/jdk1.8.0_221.jdk/Contents/Home/jre/lib/management-agent.jar:
/Library/Java/JavaVirtualMachines/jdk1.8.0_221.jdk/Contents/Home/jre/lib/plugin.jar:
/Library/Java/JavaVirtualMachines/jdk1.8.0_221.jdk/Contents/Home/jre/lib/resources.jar:
/Library/Java/JavaVirtualMachines/jdk1.8.0_221.jdk/Contents/Home/jre/lib/rt.jar:
/Library/Java/JavaVirtualMachines/jdk1.8.0_221.jdk/Contents/Home/lib/ant-javafx.jar:
/Library/Java/JavaVirtualMachines/jdk1.8.0_221.jdk/Contents/Home/lib/dt.jar:
/Library/Java/JavaVirtualMachines/jdk1.8.0_221.jdk/Contents/Home/lib/javafx-mx.jar:
/Library/Java/JavaVirtualMachines/jdk1.8.0_221.jdk/Contents/Home/lib/jconsole.jar:
/Library/Java/JavaVirtualMachines/jdk1.8.0_221.jdk/Contents/Home/lib/packager.jar:
/Library/Java/JavaVirtualMachines/jdk1.8.0_221.jdk/Contents/Home/lib/sa-jdi.jar:
/Library/Java/JavaVirtualMachines/jdk1.8.0_221.jdk/Contents/Home/lib/tools.jar:
/Users/zj/workspace/java/study/jdk8/target/classes:
/Applications/IntelliJ IDEA.app/Contents/lib/idea_rt.jar
如果使用java com.zj.study.jvm.classloader.MyTest18 在控制臺(tái)運(yùn)行，輸出結(jié)果如下
啟動(dòng)類加載器
/Library/Java/JavaVirtualMachines/jdk1.8.0_221.jdk/Contents/Home/jre/lib/resources.jar:
/Library/Java/JavaVirtualMachines/jdk1.8.0_221.jdk/Contents/Home/jre/lib/rt.jar:
/Library/Java/JavaVirtualMachines/jdk1.8.0_221.jdk/Contents/Home/jre/lib/sunrsasign.jar:
/Library/Java/JavaVirtualMachines/jdk1.8.0_221.jdk/Contents/Home/jre/lib/jsse.jar:
/Library/Java/JavaVirtualMachines/jdk1.8.0_221.jdk/Contents/Home/jre/lib/jce.jar:
/Library/Java/JavaVirtualMachines/jdk1.8.0_221.jdk/Contents/Home/jre/lib/charsets.jar:
/Library/Java/JavaVirtualMachines/jdk1.8.0_221.jdk/Contents/Home/jre/lib/jfr.jar:
/Library/Java/JavaVirtualMachines/jdk1.8.0_221.jdk/Contents/Home/jre/classes
擴(kuò)展類加載器
/Users/zj/Library/Java/Extensions:
/Library/Java/JavaVirtualMachines/jdk1.8.0_221.jdk/Contents/Home/jre/lib/ext:
/Library/Java/Extensions:
/Network/Library/Java/Extensions:
/System/Library/Java/Extensions:
/usr/lib/java
.
可以看到系統(tǒng)類加載器加載的不同彰亥，這是因?yàn)閕dea對(duì)classpath進(jìn)行修改
對(duì)于啟動(dòng)類加載器,修改啟動(dòng)類加載器加載的目錄泪漂，如
java -Dsun.boot.class.path=. com.zj.study.jvm.classloader.MyTest18
則會(huì)出現(xiàn)如下異常
Error occurred during initialization of VM
java/lang/NoClassDefFoundError: java/lang/Object
這是因?yàn)樵贠racle的Hotspot實(shí)現(xiàn)中廊营，系統(tǒng)屬性sun.boot.class.path如果設(shè)置錯(cuò)誤，則運(yùn)行會(huì)出錯(cuò)萝勤，提示如下錯(cuò)誤信息
Error occurred during initialization of VM
java/lang/NoClassDefFoundError: java/lang/Object
對(duì)于擴(kuò)展類加載器,如果動(dòng)態(tài)指定擴(kuò)展類加載器加載的目錄,比如代碼
public class MyTest19 {
public static void main(String[] args) {
AESKeyGenerator aesKeyGenerator = new AESKeyGenerator();
System.out.println(aesKeyGenerator.getClass().getClassLoader());
System.out.println(MyTest19.class.getClassLoader());
}
}
AESKeyGenerator 是由擴(kuò)展類加載器加載的
如果動(dòng)態(tài)指定擴(kuò)展類加載器加載的目錄露筒，使用如下命令運(yùn)行 會(huì)報(bào)錯(cuò)
java -Djava.ext.dirs=./ com.zj.study.jvm.classloader.MyTest19
拋java.lang.ClassNotFoundException異常
