這兩天有客戶跟我說了個問題，說他們發現weblogic不停的load class，最後線程都掛在了Zip Entry操作上。讓他們做了thread dump, 開始以為跟JDK的IO性能有關系，因為我曾經在HP\AIX上都碰到過線程掛起在zip操作上的問題，最終客戶通過調整OS參數後，問題得到解決。但在拿到thread dump後， 發現問題不是他們說的那樣，thread trace如下：
"ExecuteThread: '6' for queue: 'Out.Thread Pool'" daemon prio=5 tid=0005ff90 nid=54 lwp_id=1885955 waiting for monitor entry [0x239fa000..0x239f94f0]
at java.util.zip.ZipFile.getEntry(ZipFile.java:161)
- waiting to lock <35da8e78> (a sun.net.www.protocol.jar.URLJarFile)
at java.util.jar.JarFile.getEntry(JarFile.java:194)
at sun.net.www.protocol.jar.URLJarFile.getEntry(URLJarFile.java:89)
at sun.net.www.protocol.jar.JarURLConnection.connect(JarURLConnection.java:95)
at sun.net.www.protocol.jar.JarURLConnection.getInputStream(JarURLConnection.java:107)
at java.net.URL.openStream(URL.java:913)
at java.lang.ClassLoader.getResourceAsStream(ClassLoader.java:997)
at javax.xml.parsers.FactoryFinder.find(FactoryFinder.java:160)
at javax.xml.parsers.SAXParserFactory.newInstance(SAXParserFactory.java:87)
at org.dom4j.io.JAXPHelper.createXMLReader(JAXPHelper.java:46)
at org.dom4j.io.SAXHelper.createXMLReaderViaJAXP(SAXHelper.java:125)
at org.dom4j.io.SAXHelper.createXMLReader(SAXHelper.java:78)
at org.dom4j.io.SAXReader.createXMLReader(SAXReader.java:894)
at org.dom4j.io.SAXReader.getXMLReader(SAXReader.java:715)
at org.dom4j.io.SAXReader.read(SAXReader.java:435)
at org.dom4j.DocumentHelper.parseText(DocumentHelper.java:278)
......
我們可以看看SAXParserFactory.newInstance()的實現，
1 public static SAXParserFactory newInstance()
2 throws FactoryConfigurationError
3 {
4 try
5 {
6 return (SAXParserFactory)FactoryFinder.find("javax.xml.parsers.SAXParserFactory", "org.apache.xerces.jaxp.SAXParserFactoryImpl");
7 }
8 catch(FactoryFinder.ConfigurationError configurationerror)
9 {
10 throw new FactoryConfigurationError(configurationerror.getException(), configurationerror.getMessage());
11 }
12 }
再來看看FactoryFinder.find()的實現，
1 static Object find(String s, String s1)
2 throws ConfigurationError
3 {
4 debugPrintln("debug is on");
5 ClassLoader classloader = findClassLoader();
6 try
7 {
8 String s2 = System.getProperty(s);
9 if(s2 != null)
10 {
11 debugPrintln("found system property " + s2);
12 return newInstance(s2, classloader);
13 }
14 }
15 catch(SecurityException _ex) { }
16 try
17 {
18 String s3 = System.getProperty("java.home");
19 String s5 = s3 + File.separator + "lib" + File.separator + "jaxp.properties";
20 File file = new File(s5);
21 if(file.exists())
22 {
23 Properties properties = new Properties();
24 properties.load(new FileInputStream(file));
25 String s7 = properties.getProperty(s);
26 debugPrintln("found java.home property " + s7);
27 return newInstance(s7, classloader);
28 }
29 }
30 catch(Exception exception) { }
31 String s4 = "META-INF/services/" + s;
32 try
33 {
34 java.io.InputStream inputstream = null;
35 if(classloader == null)
36 inputstream = ClassLoader.getSystemResourceAsStream(s4);
37 else
38 inputstream = classloader.getResourceAsStream(s4);
39 if(inputstream != null)
40 {
41 debugPrintln("found " + s4);
42 BufferedReader bufferedreader;
43 try
44 {
45 bufferedreader = new BufferedReader(new InputStreamReader(inputstream, "UTF-8"));
46 }
47 catch(UnsupportedEncodingException _ex)
48 {
49 bufferedreader = new BufferedReader(new InputStreamReader(inputstream));
50 }
51 String s6 = bufferedreader.readLine();
52 bufferedreader.close();
53 if(s6 != null && !"".equals(s6))
54 {
55 debugPrintln("loaded from services: " + s6);
56 return newInstance(s6, classloader);
57 }
58 }
59 }
60 catch(Exception exception1) { }
61 if(s1 == null)
62 {
63 throw new ConfigurationError("Provider for " + s + " cannot be found", null);
64 } else
65 {
66 debugPrintln("loaded from fallback value: " + s1);
67 return newInstance(s1, classloader);
68 }
69 }
70
看看兩個方法的具體實現，這個問題基本一目了然了吧。 在我們要解析XML文件的時候，我們首先需要從parser factory獲取一個parser實例，但parser factroy是什麼？ 我們需要去查找。仔細看一下這個查找順序：
1：System property, JVM中是否定義了javax.xml.parsers.SAXParserFactory，這個可以通過-D設定
2：$JRE_HOME/ lib/jaxp.properties文件，是否存在javax.xml.parsers.SAXParserFactory鍵值對
3：xerces.jar文件的meta-inf/services/javax.xml.parsers.SAXParserFactory文件是否有值
4：當前classloader，是否存在org.apache.xerces.jaxp.SAXParserFactoryImpl，
從thread dump可以看到，客戶線程掛在了3上，因為1, 2條件不成立，所以走到了3， 而3每次都要打開jar文件，最終看到線程都停在了java.util.zip.ZipFile.getEntry()上。 解決方法：
1：啟動JVM的時候，增加-Djavax.xml.parsers.SAXParserFactory=***
2:：在$JRE_HOME/lib/增加jaxp.properties文件，其中定義javax.xml.parsers.SAXParserFactory=***
3：用展開的xerces.jar文件代替原有的jar文件
客戶同時提及了另外一個問題：我們有自己的parser factory, 為什麼最後用了weblogic的？ 而且不停的load class? Trace 如下：
"ExecuteThread: '42' for queue: 'OCS.Thread.Pool'" daemon prio=5 tid=004ca430 nid=245 lwp_id=1886146 waiting for monitor entry [0x1d9ba000..0x1d9ba4f0]
at java.lang.ClassLoader.loadClass(ClassLoader.java:278)
- waiting to lock <364cdf18> (a weblogic.utils.classloaders.GenericClassLoader)
at java.lang.ClassLoader.loadClass(ClassLoader.java:235)
at weblogic.utils.classloaders.GenericClassLoader.loadClass(GenericClassLoader.java:224)
at javax.xml.parsers.FactoryFinder.newInstance(FactoryFinder.java:93)
at javax.xml.parsers.FactoryFinder.find(FactoryFinder.java:174)
......
其實在上面的分析中可以看到，前三項中都沒有找到parser factory, 那我們只能在當前class loader中去查找，並創建instance。因為你的應用部署在weblogic上, weblogic的classloader機制是，所有的應用class loader都繼承於weblogic application class loader, 所以你會看到weblogic class loader在trace中。但為什麼不停的在class loader呢？其實看看JDK文檔，並看看JDK的實現就能知道，
protected Class<?> loadClass(String name,
boolean resolve)
throws ClassNotFoundException
Loads the class with the specified binary name. The default implementation of this method searches for classes in the following order:
Invoke findLoadedClass(String) to check if the class has already been loaded.
Invoke the loadClass method on the parent class loader. If the parent is null the class loader built-in to the virtual machine is used, instead.
Invoke the findClass(String) method to find the class.
If the class was found using the above steps, and the resolve flag is true, this method will then invoke the resolveClass(Class) method on the resulting Class object.
1 protected synchronized Class loadClass(String name, boolean resolve)
2 throws ClassNotFoundException
3 {
4 // First, check if the class has already been loaded
5 Class c = findLoadedClass(name);
6 if (c == null) {
7 try {
8 if (parent != null) {
9 c = parent.loadClass(name, false);
10 } else {
11 c = findBootstrapClass0(name);
12 }
13 } catch (ClassNotFoundException e) {
14 // If still not found, then invoke findClass in order
15 // to find the class.
16 c = findClass(name);
17 }
18 }
19 if (resolve) {
20 resolveClass(c);
21 }
22 return c;
23 }
看看，其實能看到loadclass()，並不能說class loader一定在load class，更多的時候，他們是在find loaded class，否則怎麼去創建對象？
