在 org.apache.catalina.core.JreMemoryLeakPreventionListener 中急切调用 URLConnection 的 setDefaultUseCaches(false) 的原因是什么 [英] What is the reason that setDefaultUseCaches(false) of URLConnection is eagerly called in the org.apache.catalina.core.JreMemoryLeakPreventionListener

问题描述

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 Policy.getPolicy() 的原因是什么，因为它会保留对上下文的静态引用并可能导致内存泄漏.您可以阅读它，以便了解更多背景信息.

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.

从 org.apache.cxf.common.logging.JDKBugHacks 也来自 <代码>org.apache.catalina.core.JreMemoryLeakPreventionListener.

有一段代码.在这里.

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()
 */

但是我很难理解.为什么他们急切地调用 setDefaultUseCaches(false) 以及为什么在 Windows 上默认缓存为 true 是有害的?我在 java.net.JarURLConnection.

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

我自己找到了答案.如果你认为我错了，任何人都可以纠正我.在 sun.net.www.protocol.jar.JarURLConnection.我假设这是 java.net.JarURLConnection 的默认实现.下面有一段代码.

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，则不会关闭JarFile 的连接.这意味着它被锁定了.

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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