api - 在 org.apache.catalina.core.JreMemoryLeakPreventionListener 中急切调用 URLConnection 的 setDefaultUseCaches(false) 的原因是什么

Question

这个问题可能有点难找答案。这是一个系列中的问题 What is the reason that Policy.getPolicy() is considered as it will retain a static reference to the context and can cause memory leak .您可以阅读它，以便了解更多背景。

从 org.apache.cxf.common.logging.JDKBugHacks 中提取源代码还有来自 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。为什么他们热切地调用 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 中找不到任何线索.

Answer 1

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 .我假设这是 java.net.JarURLConnection 的默认实现。下面有一段代码。

如果缓存设置为真，那么它不会关闭 JarFile 的连接。这意味着它已锁定。

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;
        }
    }