tomcat类加载器WebappClassLoader

谁借莪１个温暖的怀抱￠ 2022-12-21 02:26 196阅读 0赞

### 类/资源加载优先顺序：web-inf/classes下优先： ###

这里的类是.class文件，资源是指配置文件，比如在web.xml文件中配置的spring配置文件：

<context-param>
        <param-name>contextConfigLocation</param-name>
            <param-value>
                
                classpath:spring/spring-dao.xml
                classpath:spring/spring-service.xml
                classpath:spring/spring-dubbo*.xml
                classpath:spring/spring-quartz*.xml
                classpath:cis/spring-nacos.xml
            </param-value>
      </context-param>

tomcat类加载机制打破了jvm规范中的双亲委托机制，tomcat下各应用下的类而是由WebappClassLoader率先加载。

public class WebappClassLoader extends WebappClassLoaderBase {
    .....
    }

WebappClassLoader类继承自WebappClassLoaderBase，在WebappClassLoaderBase的getResourceAsStream方法中展现了tomcat的加载机制：

@Override
        public InputStream getResourceAsStream(String name) {
    
            if (log.isDebugEnabled())
                log.debug("getResourceAsStream(" + name + ")");
    
            checkStateForResourceLoading(name);
    
            InputStream stream = null;
    
            //从缓存加载
            // (0) Check for a cached copy of this resource
            stream = findLoadedResource(name);
            if (stream != null) {
                if (log.isDebugEnabled())
                    log.debug("  --> Returning stream from cache");
                return (stream);
            }
    
            boolean delegateFirst = delegate || filter(name);
    
            //由父加载器加载
            // (1) Delegate to parent if requested
            if (delegateFirst) {
                if (log.isDebugEnabled())
                    log.debug("  Delegating to parent classloader " + parent);
                stream = parent.getResourceAsStream(name);
                if (stream != null) {
                    // FIXME - cache???
                    if (log.isDebugEnabled())
                        log.debug("  --> Returning stream from parent");
                    return (stream);
                }
            }
    
            //由自己加载
            // (2) Search local repositories
            if (log.isDebugEnabled())
                log.debug("  Searching local repositories");
            URL url = findResource(name);
            if (url != null) {
                // FIXME - cache???
                if (log.isDebugEnabled())
                    log.debug("  --> Returning stream from local");
                stream = findLoadedResource(name);
                try {
                    if (hasExternalRepositories && (stream == null))
                        stream = url.openStream();
                } catch (IOException e) {
                    // Ignore
                }
                if (stream != null)
                    return (stream);
            }
    
            //由父加载器加载
            // (3) Delegate to parent unconditionally
            if (!delegateFirst) {
                if (log.isDebugEnabled())
                    log.debug("  Delegating to parent classloader unconditionally " + parent);
                stream = parent.getResourceAsStream(name);
                if (stream != null) {
                    // FIXME - cache???
                    if (log.isDebugEnabled())
                        log.debug("  --> Returning stream from parent");
                    return (stream);
                }
            }
        
            // (4) Resource was not found
            if (log.isDebugEnabled())
                log.debug("  --> Resource not found, returning null");
            return (null);
    
        }

由上边的源码可以看出：tomcat的WebappClassLoader加载器的加载机制：

### ①从缓存加载 ###

### ②缓存找不到时，由父加载器加载（这里也不是全部） ###

### ③父加载器加载不到时，由自己加载 ###

### ④自己加载不到时，再由父加载器加载 ###

在第②种，提到"不是全部"，这里有个判断：

protected final Matcher packageTriggersDeny = Pattern.compile(
                "^javax(\\.|/)el(\\.|/)|" +
                "^javax(\\.|/)servlet(\\.|/)|" +
                "^javax(\\.|/)websocket(\\.|/)|" +
                "^org(\\.|/)apache(\\.|/)(catalina|coyote|el|jasper|juli|naming|tomcat)(\\.|/)"
                ).matcher("");
    
    
        /**
         * Regular expression of package names which are allowed to be loaded from a
         * webapp class loader without delegating first and override any set by
         * {@link #packageTriggersDeny}.
         */
        protected final Matcher packageTriggersPermit =
                Pattern.compile("^javax(\\.|/)servlet(\\.|/)jsp(\\.|/)jstl(\\.|/)|" +
                        "^org(\\.|/)apache(\\.|/)tomcat(\\.|/)jdbc(\\.|/)").matcher("");
    protected synchronized boolean filter(String name) {
    
            if (name == null)
                return false;
    
            // Looking up the package
            String packageName = null;
            int pos = name.lastIndexOf('.');
            if (pos != -1)
                // Package names in the filters include the last '.'
                packageName = name.substring(0, pos + 1);
            else
                return false;
    
            packageTriggersPermit.reset(packageName);
            if (packageTriggersPermit.lookingAt()) {
                return false;
            }
    
            packageTriggersDeny.reset(packageName);
            if (packageTriggersDeny.lookingAt()) {
                return true;
            }
    
            return false;
        }

关于tomcat类加载器的文章：

[https://www.cnblogs.com/aspirant/p/8991830.html][https_www.cnblogs.com_aspirant_p_8991830.html]

[https://mp.weixin.qq.com/s/U16h0njbAk6aMC27S\_XDZA][https_mp.weixin.qq.com_s_U16h0njbAk6aMC27S_XDZA]

[https_www.cnblogs.com_aspirant_p_8991830.html]: https://www.cnblogs.com/aspirant/p/8991830.html
[https_mp.weixin.qq.com_s_U16h0njbAk6aMC27S_XDZA]: https://mp.weixin.qq.com/s/U16h0njbAk6aMC27S_XDZA