目录
JVM知识点划分
- class文件结构
- classLoader
- jvm运行时数据区
- 垃圾回收器及垃圾回收算法
- jit
定义
ClassLoader 顾名思义就是类加载器,ClassLoader 作用:
- 负责将 Class 加载到 JVM 中
- 审查每个类由谁加载(父优先的等级加载机制)
- 将 Class 字节码重新解析成 JVM 统一要求的对象格式
类加载器分类
系统级别
- 启动类加载器(C实现)
- 扩展类加载器(java实现)
- 系统类加载器(APP类加载器)
用户级别
自定义类加载器,集成自ClassLoader
层级结构
代码实现
启动类加载器
public static void BootStrapClassLoader() {
//获取启动类的加载器的加载目录
String bootStrapClassLoaderPath = System.getProperty("sun.boot.class.path");
//把加载后的目录转化为集合
List<String> bootStrapClassLoaderPathList = Arrays.asList(bootStrapClassLoaderPath.split(";"));
for (String path : bootStrapClassLoaderPathList) {
System.out.println("[启动类加载器-加载目录]" + path);
}
}
输出:
[启动类加载器-加载目录]C:\Program Files\Java\jdk1.8.0_91\jre\lib\resources.jar
[启动类加载器-加载目录]C:\Program Files\Java\jdk1.8.0_91\jre\lib\rt.jar
[启动类加载器-加载目录]C:\Program Files\Java\jdk1.8.0_91\jre\lib\sunrsasign.jar
[启动类加载器-加载目录]C:\Program Files\Java\jdk1.8.0_91\jre\lib\jsse.jar
[启动类加载器-加载目录]C:\Program Files\Java\jdk1.8.0_91\jre\lib\jce.jar
[启动类加载器-加载目录]C:\Program Files\Java\jdk1.8.0_91\jre\lib\charsets.jar
[启动类加载器-加载目录]C:\Program Files\Java\jdk1.8.0_91\jre\lib\jfr.jar
[启动类加载器-加载目录]C:\Program Files\Java\jdk1.8.0_91\jre\classes
扩展类加载器
public static void ExtClassLoader() {
//获取扩展类的加载器的加载目录
String extClassLoaderPath = System.getProperty("java.ext.dirs");
//把加载后的目录转化为集合
List<String> extClassLoaderPathList = Arrays.asList(extClassLoaderPath.split(";"));
for (String path : extClassLoaderPathList) {
System.out.println("[扩展类加载器-加载目录]" + path);
}
}
输出:
[扩展类加载器-加载目录]C:\Program Files\Java\jdk1.8.0_91\jre\lib\ext
[扩展类加载器-加载目录]C:\WINDOWS\Sun\Java\lib\ext
应用类加载器
public static void APPClassLoader() {
//应用类的加载器的加载目录
String appClassLoaderPath = System.getProperty("java.class.path");
//把加载后的目录转化为集合
List<String> appClassLoaderPathList = Arrays.asList(appClassLoaderPath.split(";"));
for (String path : appClassLoaderPathList) {
System.out.println("[应用类加载器-加载目录]" + path);
}
}
输出
[应用类加载器-加载目录]C:\Program Files\Java\jdk1.8.0_91\jre\lib\charsets.jar
[应用类加载器-加载目录]C:\Program Files\Java\jdk1.8.0_91\jre\lib\deploy.jar
[应用类加载器-加载目录]C:\Program Files\Java\jdk1.8.0_91\jre\lib\ext\access-bridge-64.jar
[应用类加载器-加载目录]C:\Program Files\Java\jdk1.8.0_91\jre\lib\ext\cldrdata.jar
[应用类加载器-加载目录]C:\Program Files\Java\jdk1.8.0_91\jre\lib\ext\dnsns.jar
[应用类加载器-加载目录]C:\Program Files\Java\jdk1.8.0_91\jre\lib\ext\jaccess.jar
[应用类加载器-加载目录]C:\Program Files\Java\jdk1.8.0_91\jre\lib\ext\jfxrt.jar
[应用类加载器-加载目录]C:\Program Files\Java\jdk1.8.0_91\jre\lib\ext\localedata.jar
[应用类加载器-加载目录]C:\Program Files\Java\jdk1.8.0_91\jre\lib\ext\nashorn.jar
[应用类加载器-加载目录]C:\Program Files\Java\jdk1.8.0_91\jre\lib\ext\ojdbc6-11.2.0.3.jar
[应用类加载器-加载目录]C:\Program Files\Java\jdk1.8.0_91\jre\lib\ext\sunec.jar
[应用类加载器-加载目录]C:\Program Files\Java\jdk1.8.0_91\jre\lib\ext\sunjce_provider.jar
[应用类加载器-加载目录]C:\Program Files\Java\jdk1.8.0_91\jre\lib\ext\sunmscapi.jar
[应用类加载器-加载目录]C:\Program Files\Java\jdk1.8.0_91\jre\lib\ext\sunpkcs11.jar
[应用类加载器-加载目录]C:\Program Files\Java\jdk1.8.0_91\jre\lib\ext\zipfs.jar
[应用类加载器-加载目录]C:\Program Files\Java\jdk1.8.0_91\jre\lib\javaws.jar
[应用类加载器-加载目录]C:\Program Files\Java\jdk1.8.0_91\jre\lib\jce.jar
[应用类加载器-加载目录]C:\Program Files\Java\jdk1.8.0_91\jre\lib\jfr.jar
[应用类加载器-加载目录]C:\Program Files\Java\jdk1.8.0_91\jre\lib\jfxswt.jar
[应用类加载器-加载目录]C:\Program Files\Java\jdk1.8.0_91\jre\lib\jsse.jar
[应用类加载器-加载目录]C:\Program Files\Java\jdk1.8.0_91\jre\lib\management-agent.jar
[应用类加载器-加载目录]C:\Program Files\Java\jdk1.8.0_91\jre\lib\plugin.jar
[应用类加载器-加载目录]C:\Program Files\Java\jdk1.8.0_91\jre\lib\resources.jar
[应用类加载器-加载目录]C:\Program Files\Java\jdk1.8.0_91\jre\lib\rt.jar
[应用类加载器-加载目录]C:\work\demo\jxs-parent\jxs-user\target\classes
[应用类加载器-加载目录]C:\work\demo\jxs-parent\jxs-common\jxs-common-base\target\classes
[应用类加载器-加载目录]C:\work\maven\Repositories\org\springframework\boot\spring-boot-starter-web\2.2.5.RELEASE\spring-boot-starter-web-2.2.5.RELEASE.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\org\springframework\boot\spring-boot-starter\2.2.5.RELEASE\spring-boot-starter-2.2.5.RELEASE.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\org\springframework\boot\spring-boot\2.2.5.RELEASE\spring-boot-2.2.5.RELEASE.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\org\springframework\boot\spring-boot-starter-logging\2.2.5.RELEASE\spring-boot-starter-logging-2.2.5.RELEASE.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\ch\qos\logback\logback-classic\1.2.3\logback-classic-1.2.3.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\ch\qos\logback\logback-core\1.2.3\logback-core-1.2.3.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\org\apache\logging\log4j\log4j-to-slf4j\2.12.1\log4j-to-slf4j-2.12.1.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\org\apache\logging\log4j\log4j-api\2.12.1\log4j-api-2.12.1.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\org\slf4j\jul-to-slf4j\1.7.30\jul-to-slf4j-1.7.30.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\jakarta\annotation\jakarta.annotation-api\1.3.5\jakarta.annotation-api-1.3.5.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\org\springframework\spring-core\5.2.4.RELEASE\spring-core-5.2.4.RELEASE.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\org\springframework\spring-jcl\5.2.4.RELEASE\spring-jcl-5.2.4.RELEASE.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\org\yaml\snakeyaml\1.25\snakeyaml-1.25.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\org\springframework\boot\spring-boot-starter-json\2.2.5.RELEASE\spring-boot-starter-json-2.2.5.RELEASE.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\com\fasterxml\jackson\core\jackson-databind\2.10.2\jackson-databind-2.10.2.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\com\fasterxml\jackson\core\jackson-core\2.10.2\jackson-core-2.10.2.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\com\fasterxml\jackson\datatype\jackson-datatype-jdk8\2.10.2\jackson-datatype-jdk8-2.10.2.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\com\fasterxml\jackson\datatype\jackson-datatype-jsr310\2.10.2\jackson-datatype-jsr310-2.10.2.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\com\fasterxml\jackson\module\jackson-module-parameter-names\2.10.2\jackson-module-parameter-names-2.10.2.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\org\springframework\boot\spring-boot-starter-tomcat\2.2.5.RELEASE\spring-boot-starter-tomcat-2.2.5.RELEASE.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\org\apache\tomcat\embed\tomcat-embed-core\9.0.31\tomcat-embed-core-9.0.31.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\org\apache\tomcat\embed\tomcat-embed-el\9.0.31\tomcat-embed-el-9.0.31.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\org\apache\tomcat\embed\tomcat-embed-websocket\9.0.31\tomcat-embed-websocket-9.0.31.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\org\springframework\boot\spring-boot-starter-validation\2.2.5.RELEASE\spring-boot-starter-validation-2.2.5.RELEASE.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\jakarta\validation\jakarta.validation-api\2.0.2\jakarta.validation-api-2.0.2.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\org\hibernate\validator\hibernate-validator\6.0.18.Final\hibernate-validator-6.0.18.Final.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\org\springframework\spring-web\5.2.4.RELEASE\spring-web-5.2.4.RELEASE.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\org\springframework\spring-beans\5.2.4.RELEASE\spring-beans-5.2.4.RELEASE.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\org\springframework\spring-webmvc\5.2.4.RELEASE\spring-webmvc-5.2.4.RELEASE.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\org\springframework\spring-aop\5.2.4.RELEASE\spring-aop-5.2.4.RELEASE.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\org\springframework\spring-context\5.2.4.RELEASE\spring-context-5.2.4.RELEASE.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\org\springframework\spring-expression\5.2.4.RELEASE\spring-expression-5.2.4.RELEASE.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\org\springframework\boot\spring-boot-starter-actuator\2.2.5.RELEASE\spring-boot-starter-actuator-2.2.5.RELEASE.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\org\springframework\boot\spring-boot-actuator-autoconfigure\2.2.5.RELEASE\spring-boot-actuator-autoconfigure-2.2.5.RELEASE.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\org\springframework\boot\spring-boot-actuator\2.2.5.RELEASE\spring-boot-actuator-2.2.5.RELEASE.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\io\micrometer\micrometer-core\1.3.5\micrometer-core-1.3.5.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\org\hdrhistogram\HdrHistogram\2.1.11\HdrHistogram-2.1.11.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\org\latencyutils\LatencyUtils\2.0.3\LatencyUtils-2.0.3.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\org\springframework\boot\spring-boot-starter-data-jpa\2.2.5.RELEASE\spring-boot-starter-data-jpa-2.2.5.RELEASE.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\org\springframework\boot\spring-boot-starter-aop\2.2.5.RELEASE\spring-boot-starter-aop-2.2.5.RELEASE.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\org\aspectj\aspectjweaver\1.9.5\aspectjweaver-1.9.5.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\org\springframework\boot\spring-boot-starter-jdbc\2.2.5.RELEASE\spring-boot-starter-jdbc-2.2.5.RELEASE.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\com\zaxxer\HikariCP\3.4.2\HikariCP-3.4.2.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\org\springframework\spring-jdbc\5.2.4.RELEASE\spring-jdbc-5.2.4.RELEASE.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\jakarta\activation\jakarta.activation-api\1.2.2\jakarta.activation-api-1.2.2.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\jakarta\persistence\jakarta.persistence-api\2.2.3\jakarta.persistence-api-2.2.3.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\jakarta\transaction\jakarta.transaction-api\1.3.3\jakarta.transaction-api-1.3.3.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\org\hibernate\hibernate-core\5.4.12.Final\hibernate-core-5.4.12.Final.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\org\jboss\logging\jboss-logging\3.4.1.Final\jboss-logging-3.4.1.Final.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\org\javassist\javassist\3.24.0-GA\javassist-3.24.0-GA.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\net\bytebuddy\byte-buddy\1.10.8\byte-buddy-1.10.8.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\antlr\antlr\2.7.7\antlr-2.7.7.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\org\jboss\jandex\2.1.1.Final\jandex-2.1.1.Final.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\org\dom4j\dom4j\2.1.1\dom4j-2.1.1.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\org\hibernate\common\hibernate-commons-annotations\5.1.0.Final\hibernate-commons-annotations-5.1.0.Final.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\org\glassfish\jaxb\jaxb-runtime\2.3.2\jaxb-runtime-2.3.2.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\jakarta\xml\bind\jakarta.xml.bind-api\2.3.2\jakarta.xml.bind-api-2.3.2.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\org\glassfish\jaxb\txw2\2.3.2\txw2-2.3.2.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\com\sun\istack\istack-commons-runtime\3.0.8\istack-commons-runtime-3.0.8.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\org\jvnet\staxex\stax-ex\1.8.1\stax-ex-1.8.1.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\com\sun\xml\fastinfoset\FastInfoset\1.2.16\FastInfoset-1.2.16.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\org\springframework\data\spring-data-jpa\2.2.5.RELEASE\spring-data-jpa-2.2.5.RELEASE.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\org\springframework\data\spring-data-commons\2.2.5.RELEASE\spring-data-commons-2.2.5.RELEASE.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\org\springframework\spring-orm\5.2.4.RELEASE\spring-orm-5.2.4.RELEASE.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\org\springframework\spring-tx\5.2.4.RELEASE\spring-tx-5.2.4.RELEASE.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\org\springframework\spring-aspects\5.2.4.RELEASE\spring-aspects-5.2.4.RELEASE.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\mysql\mysql-connector-java\8.0.19\mysql-connector-java-8.0.19.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\com\alibaba\druid-spring-boot-starter\1.1.20\druid-spring-boot-starter-1.1.20.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\com\alibaba\druid\1.1.20\druid-1.1.20.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\org\slf4j\slf4j-api\1.7.30\slf4j-api-1.7.30.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\org\springframework\boot\spring-boot-autoconfigure\2.2.5.RELEASE\spring-boot-autoconfigure-2.2.5.RELEASE.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\io\springfox\springfox-swagger2\2.9.2\springfox-swagger2-2.9.2.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\io\swagger\swagger-annotations\1.5.20\swagger-annotations-1.5.20.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\io\swagger\swagger-models\1.5.20\swagger-models-1.5.20.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\com\fasterxml\jackson\core\jackson-annotations\2.10.2\jackson-annotations-2.10.2.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\io\springfox\springfox-spi\2.9.2\springfox-spi-2.9.2.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\io\springfox\springfox-core\2.9.2\springfox-core-2.9.2.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\io\springfox\springfox-schema\2.9.2\springfox-schema-2.9.2.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\io\springfox\springfox-swagger-common\2.9.2\springfox-swagger-common-2.9.2.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\io\springfox\springfox-spring-web\2.9.2\springfox-spring-web-2.9.2.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\com\google\guava\guava\28.2-android\guava-28.2-android.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\com\google\guava\failureaccess\1.0.1\failureaccess-1.0.1.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\com\google\guava\listenablefuture\9999.0-empty-to-avoid-conflict-with-guava\listenablefuture-9999.0-empty-to-avoid-conflict-with-guava.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\com\google\code\findbugs\jsr305\3.0.2\jsr305-3.0.2.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\org\checkerframework\checker-compat-qual\2.5.5\checker-compat-qual-2.5.5.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\com\google\errorprone\error_prone_annotations\2.3.4\error_prone_annotations-2.3.4.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\com\google\j2objc\j2objc-annotations\1.3\j2objc-annotations-1.3.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\com\fasterxml\classmate\1.5.1\classmate-1.5.1.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\org\springframework\plugin\spring-plugin-core\1.2.0.RELEASE\spring-plugin-core-1.2.0.RELEASE.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\org\springframework\plugin\spring-plugin-metadata\1.2.0.RELEASE\spring-plugin-metadata-1.2.0.RELEASE.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\org\mapstruct\mapstruct\1.2.0.Final\mapstruct-1.2.0.Final.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\io\springfox\springfox-swagger-ui\2.9.2\springfox-swagger-ui-2.9.2.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\com\baomidou\mybatis-plus-boot-starter\3.1.0\mybatis-plus-boot-starter-3.1.0.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\com\baomidou\mybatis-plus\3.1.0\mybatis-plus-3.1.0.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\com\baomidou\mybatis-plus-generator\3.1.0\mybatis-plus-generator-3.1.0.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\com\baomidou\mybatis-plus-extension\3.1.0\mybatis-plus-extension-3.1.0.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\com\baomidou\mybatis-plus-core\3.1.0\mybatis-plus-core-3.1.0.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\com\baomidou\mybatis-plus-annotation\3.1.0\mybatis-plus-annotation-3.1.0.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\org\mybatis\mybatis\3.5.0\mybatis-3.5.0.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\com\github\jsqlparser\jsqlparser\1.4\jsqlparser-1.4.jar
[应用类加载器-加载目录]C:\work\maven\Repositories\org\mybatis\mybatis-spring\2.0.0\mybatis-spring-2.0.0.jar
[应用类加载器-加载目录]C:\Program Files\JetBrains\IntelliJ IDEA 2019.2\lib\idea_rt.jar
双亲委派原则
代码实现
System.out.println("当前加载器:"+DiffClassLoader.class.getClassLoader());
System.out.println("当前加载器-父类:"+DiffClassLoader.class.getClassLoader().getParent());
System.out.println("当前加载器-父类-父类:"+DiffClassLoader.class.getClassLoader().getParent().getParent());
输出:
当前加载器:sun.misc.Launcher$AppClassLoader@18b4aac2
当前加载器-父类:sun.misc.Launcher$ExtClassLoader@37f8bb67
当前加载器-父类-父类:null
源码探索
什么时候出发类加载
- 访问类的静态变量 (除常量【 被final修辞的静态变量】 原因:常量一种特殊的变量,因为编译器把他们当作值(value)而不是域(field)来对待。如果你的代码中用到了常变量(constant variable),编译器并不会生成字节码来从对象中载入域的值,而是直接把这个值插入到字节码中。这是一种很有用的优化,但是如果你需要改变final域的值那么每一块用到那个域的代码都需要重新编译。
- 访问类的静态方法
- 虚拟机启动时,定义了main()方法的那个类先初始化
- 创建类的实例 new指令
- 反射 如( Class.forName(“my.xyz.Test”) )
- 子类初始化,一定会出发父类加载
初始化一定会类加载
类加载不一定会触发初始化
重要方法
loadClass()
/**
* 加载指定的限定名,默认实现以下方法:
* 1. 调用 findLoadedClass(String)方法来校验class是否被加载
* 2. 调用loadClass(String) 方法,在父类加载器上,如果父类加载器是null,则调用虚拟机 提供的加载器,即为BootStrapClassLoader
* 3. 调用findClass(String) 方法, 来寻找class
*
* 参数:
* name:全限定名
* resolve 如果是true 解决这个类
*
*
*
* 如果在上述步骤中找到类, 且 "resolve" 属性是true ,将会调用resolveClass(Class)
* 在最后返回class时
*
* 子类加载器鼓励去实现 findClass(String)方法
*
*/
protected Class<?> loadClass(String name, boolean resolve)
throws ClassNotFoundException
{
synchronized (getClassLoadingLock(name)) {
// 1. 校验类是否被加载
Class<?> c = findLoadedClass(name);
if (c == null) {
long t0 = System.nanoTime();
try {
if (parent != null) {
//父加载器不为空则调用父加载器的loadClass
c = parent.loadClass(name, false);
} else {
//如果为空调用启动类加载器
c = findBootstrapClassOrNull(name);
}
} catch (ClassNotFoundException e) {
// ClassNotFoundException thrown if class not found
// from the non-null parent class loader
}
if (c == null) {
//如果分类加载器没有找到,则调用findClass 来寻找类
long t1 = System.nanoTime();
c = findClass(name);
// this is the defining class loader; record the stats
sun.misc.PerfCounter.getParentDelegationTime().addTime(t1 - t0);
sun.misc.PerfCounter.getFindClassTime().addElapsedTimeFrom(t1);
sun.misc.PerfCounter.getFindClasses().increment();
}
}
if (resolve) {
调用resolveClass()
resolveClass(c);
}
return c;
}
}
加载流程
手写自定义加载器
新建需要加载的类
package com.jxs.user.classLoader;
public class Person {
public void say() {
System.out.println("Say Hello");
}
}
自定义CustomerClassLoader
package com.jxs.user.classLoader;
import java.io.*;
public class CustomerClassLoader extends ClassLoader {
private String mLibPath;
public CustomerClassLoader(String path) {
// TODO Auto-generated constructor stub
mLibPath = path;
}
/**
* Finds the class with the specified <a href="#name">binary name</a>.
* This method should be overridden by class loader implementations that
* follow the delegation model for loading classes, and will be invoked by
* the {@link #loadClass <tt>loadClass</tt>} method after checking the
* parent class loader for the requested class. The default implementation
* throws a <tt>ClassNotFoundException</tt>.
*
* @param name The <a href="#name">binary name</a> of the class
* @return The resulting <tt>Class</tt> object
* @throws ClassNotFoundException If the class could not be found
* @since 1.2
*/
@Override
protected Class<?> findClass(String name) throws ClassNotFoundException {
String fileName = getFileName(name);
//加载文件
File file = new File(mLibPath, fileName);
try {
FileInputStream is = new FileInputStream(file);
//输出流
ByteArrayOutputStream bos = new ByteArrayOutputStream();
int len = 0;
if ((len = is.read()) != -1) {
bos.write(len);
}
byte[] data = bos.toByteArray();
is.close();
bos.close();
return defineClass(name, data, 0, data.length);
} catch (FileNotFoundException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
return super.findClass(name);
}
/**
* @param name 获取要加载 的class文件名
* @return
*/
private String getFileName(String name) {
// TODO Auto-generated method stub
int index = name.lastIndexOf('.');
if (index == -1) {
return name + ".class";
} else {
return name.substring(index + 1) + ".class";
}
}
}
测试类
package com.jxs.user.classLoader;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
public class ClassLoaderTest {
public static void main(String[] args) {
// TODO Auto-generated method stub
//创建自定义classloader对象。
CustomerClassLoader diskLoader = new CustomerClassLoader("C:\\work\\demo\\jxs-parent\\jxs-user\\target\\classes");
try {
//加载class文件
Class c = diskLoader.loadClass("com.jxs.user.classLoader.Person");
if(c != null){
try {
Object obj = c.newInstance();
Method method = c.getDeclaredMethod("say",null);
//通过反射调用Test类的say方法
method.invoke(obj, null);
} catch (InstantiationException | IllegalAccessException
| NoSuchMethodException
| SecurityException |
IllegalArgumentException |
InvocationTargetException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
} catch (ClassNotFoundException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
输出:
Say Hello