org.apache.catalina.core.JreMemoryLeakPreventionListener 中急切调用 URLConnection 的 setDefaultUseCaches(false) 是什么原因

这个问题可能有点难以找到答案。这是一个系列中的问题考虑使用 Policy.getPolicy() 的原因是什么，因为它将保留对上下文的静态引用并可能导致内存泄漏 https://stackoverflow.com/questions/7057421/why-policy-getpolicy-is-considered-as-it-will-retain-a-static-reference-to-the。您可以阅读它，以便您了解更多背景。

源代码来自org.apache.cxf.common.logging.JDKBugHacks http://grepcode.com/file/repo1.maven.org/maven2/org.apache.cxf/cxf-common-utilities/2.3.1/org/apache/cxf/common/logging/JDKBugHacks.java也来自org.apache.catalina.core.JreMemoryLeakPreventionListener http://grepcode.com/file/repo1.maven.org/maven2/org.apache.tomcat/tomcat-catalina/7.0.0/org/apache/catalina/core/JreMemoryLeakPreventionListener.java.

有一段代码。这里是。

URL url = new URL("jar:file://dummy.jar!/"); 
URLConnection uConn = new URLConnection(url) {

    @Override
    public void connect() throws IOException{ 
        // NOOP
    }

};
uConn.setDefaultUseCaches(false);

评论说

/*
 * Several components end up opening JarURLConnections without
 * first disabling caching. This effectively locks the file.
 * Whilst more noticeable and harder to ignore on Windows, it
 * affects all operating systems.
 * 
 * Those libraries/components known to trigger this issue
 * include:
 * - log4j versions 1.2.15 and earlier
 * - javax.xml.bind.JAXBContext.newInstance()
 */

但我很难理解它。为什么他们急切地打电话setDefaultUseCaches(false)为什么在 Windows 上默认缓存为 true 是有害的？我找不到任何线索java.net.JarURLConnection http://grepcode.com/file/repository.grepcode.com/java/root/jdk/openjdk/6-b14/java/net/JarURLConnection.java.

我自己找到了答案。如果您认为我错了，任何人都可以纠正我。 在sun.net.www.protocol.jar.JarURLConnection http://grepcode.com/file/repository.grepcode.com/java/root/jdk/openjdk/6-b14/sun/net/www/protocol/jar/JarURLConnection.java?av=h#JarURLConnection。我认为这是默认实现java.net.JarURLConnection。下面有一段代码。

如果cache设置为true，那么它不会关闭JarFile的连接。这意味着它被锁定了。

class JarURLInputStream extends java.io.FilterInputStream {
        JarURLInputStream (InputStream src) {
            super (src);
        }
        public void close () throws IOException {
            try {
                super.close();
            } finally {
                if (!getUseCaches()) {
                    jarFile.close(); //will not close
                }
            }
        }
    }



    public void connect() throws IOException {
        if (!connected) {
            /* the factory call will do the security checks */
            jarFile = factory.get(getJarFileURL(), getUseCaches());

            /* we also ask the factory the permission that was required
             * to get the jarFile, and set it as our permission.
             */
            if (getUseCaches()) {
                jarFileURLConnection = factory.getConnection(jarFile);
            }

            if ((entryName != null)) {
                jarEntry = (JarEntry)jarFile.getEntry(entryName);
                if (jarEntry == null) {
                    try {
                        if (!getUseCaches()) {
                            jarFile.close();  //will not close
                        }
                    } catch (Exception e) {
                    }
                    throw new FileNotFoundException("JAR entry " + entryName +
                                                    " not found in " +
                                                    jarFile.getName());
                }
            }
            connected = true;
        }
    }