那是什么的URLConnection的setDefaultUseCaches（假）被称为急切地在org.apache.catalina.core.JreMemoryLeak preventionListener的原因 [英] What is the reason that setDefaultUseCaches(false) of URLConnection is eagerly called in the org.apache.catalina.core.JreMemoryLeakPreventionListener

问题描述

此问题可能是一个有点难以找到答案。这是在一个系列中的问题与<一个href=\"http://stackoverflow.com/questions/7057421/why-policy-getpolicy-is-considered-as-it-will-retain-a-static-reference-to-the\">What是Policy.getPolicy（）被认为是因为它会保留一个静态引用的上下文，并可能导致内存泄漏中的原因。可以读取它，所以你可以了解更多的背景。

This question could be a bit difficult to find the answer. It's a questions in one series with What is the reason that Policy.getPolicy() is considered as it will retain a static reference to the context and can cause memory leak. You can read it so you may know more background.

Graped从<一个源$ C ​​$ C href=\"http://grep$c$c.com/file/repo1.maven.org/maven2/org.apache.cxf/cxf-common-utilities/2.3.1/org/apache/cxf/common/logging/JDKBugHacks.java\"相对=nofollow> org.apache.cxf.common.logging.JDKBugHacks ，也从<一个href=\"http://grep$c$c.com/file/repo1.maven.org/maven2/org.apache.tomcat/tomcat-catalina/7.0.0/org/apache/catalina/core/JreMemoryLeak$p$pventionListener.java\"相对=nofollow> org.apache.catalina.core.JreMemoryLeak preventionListener 。

Graped the source code from org.apache.cxf.common.logging.JDKBugHacks and also from org.apache.catalina.core.JreMemoryLeakPreventionListener.

有是一块code的。这就是。

There is a piece of code. Here it is.

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

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

};
uConn.setDefaultUseCaches(false);

的评论称

/*
 * 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()
 */

不过，我也很难理解。为什么他们急切地叫 setDefaultUseCaches（假）为什么在Windows上是有害的，默认情况下缓存是真的吗？我找不到<一个任何线索href=\"http://grep$c$c.com/file/repository.grep$c$c.com/java/root/jdk/openjdk/6-b14/java/net/JarURLConnection.java\"相对=nofollow> java.net.JarURLConnection中 。

However I can hardly understand it. Why did they eagerly call setDefaultUseCaches(false) and why on Windows it's harmful that by default cache is true? I cannot find any clue in java.net.JarURLConnection.

推荐答案

我自己找到答案。如果你认为我错了，任何一个可以指正。
在<一个href=\"http://grep$c$c.com/file/repository.grep$c$c.com/java/root/jdk/openjdk/6-b14/sun/net/www/protocol/jar/JarURLConnection.java?av=h#JarURLConnection\"相对=nofollow> sun.net.www.protocol.jar.JarURLConnection 。我想这是 java.net.JarURLConnection中的默认实现。有一块低于code的。

I myself find an answer. Any one can correct me if you think I am wrong. in sun.net.www.protocol.jar.JarURLConnection. I assume this is the default implementation of java.net.JarURLConnection. There is a piece of code below.

如果缓存设置为true，那么就不会关闭jar文件的连接。这意味着它已被锁定。

If cache is set to true, then it will not close the JarFile's connection. Which means it is locked.

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;
        }
    }

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