b.Jre Memory Leak Prevention Listener

Enables protection so that calls to sun.awt.AppContext.getAppContext() triggered by a web application do not result in a memory leak. Note that enabling this protection will trigger a requirement for a graphical environment unless Java is started in head-less mode. The default is false. This protection is disabled if running on Java 8 onwards since the leak has been fixed for Java 8 onwards.

web应用多次执行sun.awt.AppContext.getAppContext()会造成泄露，这里需要进行一下保护，代码为：

Enables protection so that calls to java.awt.Toolkit.getDefaultToolkit() triggered by a web application do not result in a memory leak. Defaults to false because an AWT thread is launched. This protection is disabled if running on Java 9 onwards since the leak has been fixed for Java 9 onwards.

web应用多次执行java.awt.Toolkit.getDefaultToolkit() 会造成内存泄露，保护代码为：

List of comma-separated fully qualified class names to load and initialize during the startup of this Listener. This allows to pre-load classes that are known to provoke classloader leaks if they are loaded during a request processing. Non-JRE classes may be referenced, like oracle.jdbc.driver.OracleTimeoutThreadPerVM. The default value is empty, but specific JRE classes are loaded by other leak protection features managed by other attributes of this Listener.

预先通过系统类加载器加载的类，以逗号分隔，这样类就不会有多份了，相当于减少perm区的占用；

代码见上；

The first use of java.sql.DriverManager will trigger the loading of JDBC Driver in the current class loader. The web application level memory leak protection can take care of this in most cases but triggering the loading here has fewer side-effects. The default is true.

数据库驱动的初始化先搞一轮，驱动的实现就不会每一个应用一份，减少对象创建；

代码见上；

Enables protection so that calls to sun.misc.GC.requestLatency(long) triggered by a web application do not result in a memory leak. Use of RMI is likely to trigger a call to this method. A side effect of enabling this protection is the creation of a thread named "GC Daemon". The protection uses reflection to access internal Sun classes and may generate errors on startup on non-Sun JVMs. The default is true. This protection is disabled if running on Java 9 onwards since the leak has been fixed for Java 9 onwards.

作为SUN的JDK，在进行GC请求的时候，会调用sun.misc.GC.requestLatency(long)  ；

而调用完这个方法之后，SUN的实现会启动一个daemon线程；

试想一下，如果每一个应用调用这段代码的话，即很容易产生内存泄露；

Enables protection so that the PoolCleaner thread started by com.sun.jndi.ldap.LdapPoolManager does not result in a memory leak. The thread is started the first time the LdapPoolManager class is used if the system property com.sun.jndi.ldap.connect.pool.timeout is set to a value greater than 0. Without this protection, if a web application uses this class the PoolCleaner thread will be configured with the thread‘s context class loader set to the web application class loader which in turn will trigger a memory leak on reload. Defaults to true. This protection is disabled if running on Java 9 onwards since the leak has been fixed for Java 9 onwards.

Ldap的池中的PoolCleaner 线程如果每一个应用的应用classLoader加载极容易出现内存泄露；

Enables protection so that usage of the javax.security.auth.login.Configuration class by a web application does not provoke a memory leak. The first access of this class will trigger the initializer that will retain a static reference to the context class loader. The protection loads the class with the system class loader to ensure that the static initializer is not triggered by a web application. Defaults to true. This protection is disabled if running on Java 8 onwards since the leak has been fixed for Java 8 onwards.

让系统类加载器就把Configuration  初始化了，可以保证静态的initializer  不被web应用的类加载器调用，保证只有1份；

Enables protection so that usage of the deprecated javax.security.auth.Policy class by a web application does not result in a memory leak. The first access of this class will trigger the static initializer that will retain a static reference to the context class loader. The protection calls the getPolicy() method of this class to ensure that the static initializer is not triggered by a web application. Defaults to true.

Note: The underlying leak has been fixed in Java 7 update 51 onwards and Java 8 onwards. This protection is therefor disabled if running on Java 8 onwards.

Policy 的配置，和上一个securityLoginConfigurationProtection的配置一个意思；

Enables protection so that any token poller thread initialized by sun.security.pkcs11.SunPKCS11.initToken() does not result in a memory leak. The thread is started depending on various conditions as part of the initialization of the Java Cryptography Architecture. Without the protection this can happen during Webapp deployment when the MessageDigest for generating session IDs is initialized. As a result the thread has the Webapp class loader as its thread context class loader. Enabling the protection initializes JCA early during Tomcat startup. Defaults to true. This protection is disabled if running on Java 9 onwards since the leak has been fixed for Java 9 onwards.

减少Java Cryptography Architecture的线程，不创建那么多个tokenPollerProtection线程；

Enables protection so that reading resources from JAR files using java.net.URLConnections does not result in the JAR file being locked. Note that enabling this protection disables caching by default for all resources obtained via java.net.URLConnections. Caching may be re-enabled on a case by case basis as required. Defaults to true.

在读取jar文件的时候，极容易造成lock文件，而实质原因是jarConnection中缓存默认设置没有禁用；

解决办法就是找一个根本不存在的jar链接，然后设置就OK了：

Enables protection so that parsing XML files within a web application does not result in a memory leak. Note that memory profilers may not display the GC root associated with this leak making it particularly hard to diagnose. Defaults to true. This protection is disabled if running on Java 9 onwards since the leak has been fixed for Java 9 onwards.

尽可能的多初始化dom实现的内容，减少对象；