浅析Velocity加载机制
项目注册VelocityEngine的代码
velocity.properties
从前面两句代码可以知道作用是从velocity.properties配置文件加载Properties这个类中，并且以设置key和value的形式存储在Properties中，方便VelocityEngine这个类使用。
接下来看这个带一个参数的构造方法
我们可以看VelocityEngine类的部分源码
public class VelocityEngine implements RuntimeConstants {
//-- 创建RuntimeInstance单实例，使用的是单例模式
private RuntimeInstance ri = new RuntimeInstance();
public VelocityEngine() {}
public VelocityEngine(String propsFilename) {
ri.setProperties(propsFilename);
}
public VelocityEngine(Properties p) {
ri.setProperties(p);
}
public void init() {
ri.init();
}
public void init(String propsFilename) {
ri.init(propsFilename);
}
public void init(Properties p) {
ri.init(p);
}
//-- 其它部分
......
}
从上面部分代码可以看出创建VelocityEngine实例的时候，也会初始化RuntimeConstants常量接口，部分代码：
package org.apache.velocity.runtime;
public interface RuntimeConstants {
String ENCODING_DEFAULT = "ISO-8859-1";
String DEFAULT_RUNTIME_PROPERTIES = "org/apache/velocity/runtime/defaults/velocity.properties";
String DEFAULT_RUNTIME_DIRECTIVES = "org/apache/velocity/runtime/defaults/directive.properties";
int NUMBER_OF_PARSERS = 20;
}
整个常量接口就是定义默认配置文件中key值以及一些key的默认value值。
接下里也会创建一个RuntimeInstance实例，再来看整个RuntimeInstance类部分源码，至于他的父类接口都是会加载的，不再看具体父接口的代码，
package org.apache.velocity.runtime;
public class RuntimeInstance implements RuntimeConstants, RuntimeServices {
private VelocimacroFactory vmFactory = null;
private Log log = new Log();
private ParserPool parserPool;
private boolean initializing = false;
private volatile boolean initialized = false;
private ExtendedProperties overridingProperties = null;
.....
private Introspector introspector = null;
private Map applicationAttributes = null;
private String encoding;
//-- 构造方法
public RuntimeInstance() {
vmFactory = new VelocimacroFactory( this );
introspector = new Introspector(getLog());
applicationAttributes = new HashMap();
}
public Log getLog() {
return log;
}
}
这个类RuntimeInstance的实例创建完毕之后，在VelocityEngine中将新设置的属性通过RuntimeInstance的setProperties()方法设置到RuntimeInstance实例对象中存储。
以上仅仅只是设置属性内容，那么Velocity到底是怎么进行初始化的呢？
如果在Spring容器启动的时候不初始化Velocity，那么就需要自己在项目中自己写代码进行初始化
截取一段代码
String vm = "/template/debt-listing-apply.html";
VelocityContext context = new VelocityContext();
context.put("project", project);
StringWriter stringWriter = new StringWriter();
velocityEngine.mergeTemplate(vm, "UTF-8", context, stringWriter);
比较核心的两个类是VelocityContext和velocityEngine
VelocityContext部分源码：
package org.apache.velocity;
public class VelocityContext extends AbstractContext implements Cloneable {
private static final long serialVersionUID = 9033846851064645037L;
private Map context = null;
public VelocityContext() {
this(null, null);
}
public VelocityContext(Map context, Context innerContext) {
super(innerContext);
this.context = (context == null ? new HashMap() : context);
}
}
velocityEngine部分源码：
package org.apache.velocity.app;
public class VelocityEngine implements RuntimeConstants {
private RuntimeInstance ri = new RuntimeInstance();
public boolean mergeTemplate(String templateName, String encoding,
Context context, Writer writer )
throws ResourceNotFoundException, ParseErrorException, MethodInvocationException {
Template template = ri.getTemplate(templateName, encoding);
if ( template == null )
{
String msg = "VelocityEngine.mergeTemplate() was unable to load template '"
+ templateName + "'";
getLog().error(msg);
throw new ResourceNotFoundException(msg);
}
else
{
template.merge(context, writer);
return true;
}
}
}
RuntimeInstance部分源码：
package org.apache.velocity.runtime;
public class RuntimeInstance implements RuntimeConstants, RuntimeServices {
public Template getTemplate(String name, String encoding)
throws ResourceNotFoundException, ParseErrorException {
requireInitialization();
return (Template)
resourceManager.getResource(name,
ResourceManager.RESOURCE_TEMPLATE, encoding);
}
private void requireInitialization() {
if (!initialized) {
try {
init();
}
catch (Exception e) {
getLog().error("Could not auto-initialize Velocity", e);
throw new RuntimeException("Velocity could not be initialized!", e);
}
}
}
public synchronized void init() {
if (!initialized && !initializing) {
log.debug("Initializing Velocity, Calling init()...");
initializing = true;
log.trace("*******************************************************************");
log.debug("Starting Apache Velocity v1.7 (compiled: 2010-11-19 12:14:37)");
log.trace("RuntimeInstance initializing.");
initializeProperties();
initializeLog();
initializeResourceManager();
initializeDirectives();
initializeEventHandlers();
initializeParserPool();
initializeIntrospection();
initializeEvaluateScopeSettings();
/*
* initialize the VM Factory. It will use the properties
* accessable from Runtime, so keep this here at the end.
*/
vmFactory.initVelocimacro();
log.trace("RuntimeInstance successfully initialized.");
initialized = true;
initializing = false;
}
}
private void initializeProperties() {
if (configuration.isInitialized() == false) {
setDefaultProperties();
}
if( overridingProperties != null) {
configuration.combine(overridingProperties);
}
}
......
}
通过跟踪源码，知道了自己设置的属性是在什么时候加载和设置到Velocity中，又是怎么加载默认配置文件的，又是怎么进行一步一步初始化的。详细了解可以看源码。
上述的方式仅仅是在方法的调用时进行初始化，如果是在Spring容器启动的过程中进行初始化，那么就需要加载VelocityServlet和初始化，不过Spring4.3之后不再支持，需要做一些特殊的处理，在Spring初始化时也加载这个Servlet，并且初始化这个Servlet，
下面会调用：
接下来调用
接着调用
最终还是使用RuntimeInstance的init()方法进行初始化。如果需要使用Velocity模板进行页面开发，那么就需要在Spring容器加载初始化时做特殊处理。
附上默认配置文件内容
默认velocity.properties配置文件内容
# Licensed to the Apache Software Foundation (ASF) under one
# or more contributor license agreements. See the NOTICE file
# distributed with this work for additional information
# regarding copyright ownership. The ASF licenses this file
# to you under the Apache License, Version 2.0 (the
# "License"); you may not use this file except in compliance
# with the License. You may obtain a copy of the License at
#
# http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing,
# software distributed under the License is distributed on an
# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
# KIND, either express or implied. See the License for the
# specific language governing permissions and limitations
# under the License.
# ----------------------------------------------------------------------------
# R U N T I M E L O G
# ----------------------------------------------------------------------------
# ----------------------------------------------------------------------------
# default LogChute to use: default: AvalonLogChute, Log4JLogChute, CommonsLogLogChute, ServletLogChute, JdkLogChute
# ----------------------------------------------------------------------------
runtime.log.logsystem.class = org.apache.velocity.runtime.log.AvalonLogChute,org.apache.velocity.runtime.log.Log4JLogChute,org.apache.velocity.runtime.log.CommonsLogLogChute,org.apache.velocity.runtime.log.ServletLogChute,org.apache.velocity.runtime.log.JdkLogChute
# ---------------------------------------------------------------------------
# This is the location of the Velocity Runtime log.
# ----------------------------------------------------------------------------
runtime.log = velocity.log
# ----------------------------------------------------------------------------
# This controls whether invalid references are logged.
# ----------------------------------------------------------------------------
runtime.log.invalid.references = true
# ----------------------------------------------------------------------------
# T E M P L A T E E N C O D I N G
# ----------------------------------------------------------------------------
input.encoding=ISO-8859-1
output.encoding=ISO-8859-1
# ----------------------------------------------------------------------------
# F O R E A C H P R O P E R T I E S
# ----------------------------------------------------------------------------
# These properties control how the counter is accessed in the #foreach
# directive. By default the reference $velocityCount and $velocityHasNext
# will be available in the body of the #foreach directive.
# The default starting value for $velocityCount is 1.
# ----------------------------------------------------------------------------
directive.foreach.counter.name = velocityCount
directive.foreach.counter.initial.value = 1
directive.foreach.maxloops = -1
directive.foreach.iterator.name = velocityHasNext
# ----------------------------------------------------------------------------
# S E T P R O P E R T I E S
# ----------------------------------------------------------------------------
# These properties control the behavior of #set.
# For compatibility, the default behavior is to disallow setting a reference
# to null. This default may be changed in a future version.
# ----------------------------------------------------------------------------
directive.set.null.allowed = false
# ----------------------------------------------------------------------------
# I F P R O P E R T I E S
# ----------------------------------------------------------------------------
# These properties control the behavior of #if
# Default behavior is to check return value of toString() and treat an object
# with toString() that returns null as null. If all objects have toString()
# methods that never return null, this check is unnecessary and can be disabled
# to gain performance. In Velocity 1.5, no such null check was performed.
directive.if.tostring.nullcheck = true
# ----------------------------------------------------------------------------
# I N C L U D E P R O P E R T I E S
# ----------------------------------------------------------------------------
# These are the properties that governed the way #include'd content
# is governed.
# ----------------------------------------------------------------------------
directive.include.output.errormsg.start = <!-- include error :
directive.include.output.errormsg.end = see error log -->
# ----------------------------------------------------------------------------
# P A R S E P R O P E R T I E S
# ----------------------------------------------------------------------------
directive.parse.max.depth = 10
# ----------------------------------------------------------------------------
# S C O P E P R O P E R T I E S
# ----------------------------------------------------------------------------
# These are the properties that govern whether or not a Scope object
# is automatically provided for each of the given scopes to serve as a
# scope-safe reference namespace and "label" for #break calls. The default
# for most of these is false. Note that <bodymacroname> should be replaced by
# name of macros that take bodies for which you want to suppress the scope.
# ----------------------------------------------------------------------------
# template.provide.scope.control = false
# evaluate.provide.scope.control = false
foreach.provide.scope.control = true
# macro.provide.scope.control = false
# define.provide.scope.control = false
# <bodymacroname>.provide.scope.control = false
# ----------------------------------------------------------------------------
# T E M P L A T E L O A D E R S
# ----------------------------------------------------------------------------
#
#
# ----------------------------------------------------------------------------
resource.loader = file
file.resource.loader.description = Velocity File Resource Loader
file.resource.loader.class = org.apache.velocity.runtime.resource.loader.FileResourceLoader
file.resource.loader.path = .
file.resource.loader.cache = false
file.resource.loader.modificationCheckInterval = 2
string.resource.loader.description = Velocity String Resource Loader
string.resource.loader.class = org.apache.velocity.runtime.resource.loader.StringResourceLoader
# ----------------------------------------------------------------------------
# VELOCIMACRO PROPERTIES
# ----------------------------------------------------------------------------
# global : name of default global library. It is expected to be in the regular
# template path. You may remove it (either the file or this property) if
# you wish with no harm.
# ----------------------------------------------------------------------------
# velocimacro.library = VM_global_library.vm
velocimacro.permissions.allow.inline = true
velocimacro.permissions.allow.inline.to.replace.global = false
velocimacro.permissions.allow.inline.local.scope = false
velocimacro.context.localscope = false
velocimacro.max.depth = 20
# ----------------------------------------------------------------------------
# VELOCIMACRO STRICT MODE
# ----------------------------------------------------------------------------
# if true, will throw an exception for incorrect number
# of arguments. false by default (for backwards compatibility)
# but this option will eventually be removed and will always
# act as if true
# ----------------------------------------------------------------------------
velocimacro.arguments.strict = false
# ----------------------------------------------------------------------------
# VELOCIMACRO BODY REFERENCE
# ----------------------------------------------------------------------------
# Defines name of the reference that can be used to render the AST block passed to
# block macro call as an argument inside a macro.
# ----------------------------------------------------------------------------
velocimacro.body.reference=bodyContent
# ----------------------------------------------------------------------------
# STRICT REFERENCE MODE
# ----------------------------------------------------------------------------
# if true, will throw a MethodInvocationException for references
# that are not defined in the context, or have not been defined
# with a #set directive. This setting will also throw an exception
# if an attempt is made to call a non-existing property on an object
# or if the object is null. When this property is true then property
# 'directive.set.null.allowed' is also set to true.
# ----------------------------------------------------------------------------
runtime.references.strict = false
# ----------------------------------------------------------------------------
# INTERPOLATION
# ----------------------------------------------------------------------------
# turn off and on interpolation of references and directives in string
# literals. ON by default :)
# ----------------------------------------------------------------------------
runtime.interpolate.string.literals = true
# ----------------------------------------------------------------------------
# RESOURCE MANAGEMENT
# ----------------------------------------------------------------------------
# Allows alternative ResourceManager and ResourceCache implementations
# to be plugged in.
# ----------------------------------------------------------------------------
resource.manager.class = org.apache.velocity.runtime.resource.ResourceManagerImpl
resource.manager.cache.class = org.apache.velocity.runtime.resource.ResourceCacheImpl
# ----------------------------------------------------------------------------
# PARSER POOL
# ----------------------------------------------------------------------------
# Selects a custom factory class for the parser pool. Must implement
# ParserPool. parser.pool.size is used by the default implementation
# ParserPoolImpl
# ----------------------------------------------------------------------------
parser.pool.class = org.apache.velocity.runtime.ParserPoolImpl
parser.pool.size = 20
# ----------------------------------------------------------------------------
# EVENT HANDLER
# ----------------------------------------------------------------------------
# Allows alternative event handlers to be plugged in. Note that each
# class property is actually a comma-separated list of classes (which will
# be called in order).
# ----------------------------------------------------------------------------
# eventhandler.referenceinsertion.class =
# eventhandler.nullset.class =
# eventhandler.methodexception.class =
# eventhandler.include.class =
# ----------------------------------------------------------------------------
# EVALUATE
# ----------------------------------------------------------------------------
# Evaluate VTL dynamically in template. Select a class for the Context, if
# you want all #set calls within it to be locally scoped. This feature is
# deprecated; please use $evaluate to hold local references instead.
# ----------------------------------------------------------------------------
#directive.evaluate.context.class = org.apache.velocity.VelocityContext
# ----------------------------------------------------------------------------
# PLUGGABLE INTROSPECTOR
# ----------------------------------------------------------------------------
# Allows alternative introspection and all that can of worms brings.
# ----------------------------------------------------------------------------
runtime.introspector.uberspect = org.apache.velocity.util.introspection.UberspectImpl
# ----------------------------------------------------------------------------
# SECURE INTROSPECTOR
# ----------------------------------------------------------------------------
# If selected, prohibits methods in certain classes and packages from being
# accessed.
# ----------------------------------------------------------------------------
introspector.restrict.packages = java.lang.reflect
# The two most dangerous classes
introspector.restrict.classes = java.lang.Class
introspector.restrict.classes = java.lang.ClassLoader
# Restrict these for extra safety
introspector.restrict.classes = java.lang.Compiler
introspector.restrict.classes = java.lang.InheritableThreadLocal
introspector.restrict.classes = java.lang.Package
introspector.restrict.classes = java.lang.Process
introspector.restrict.classes = java.lang.Runtime
introspector.restrict.classes = java.lang.RuntimePermission
introspector.restrict.classes = java.lang.SecurityManager
introspector.restrict.classes = java.lang.System
introspector.restrict.classes = java.lang.Thread
introspector.restrict.classes = java.lang.ThreadGroup
introspector.restrict.classes = java.lang.ThreadLocal
directive.properties默认配置文件内容
# Licensed to the Apache Software Foundation (ASF) under one
# or more contributor license agreements. See the NOTICE file
# distributed with this work for additional information
# regarding copyright ownership. The ASF licenses this file
# to you under the Apache License, Version 2.0 (the
# "License"); you may not use this file except in compliance
# with the License. You may obtain a copy of the License at
#
# http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing,
# software distributed under the License is distributed on an
# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
# KIND, either express or implied. See the License for the
# specific language governing permissions and limitations
# under the License.
directive.1=org.apache.velocity.runtime.directive.Foreach
directive.2=org.apache.velocity.runtime.directive.Include
directive.3=org.apache.velocity.runtime.directive.Parse
directive.4=org.apache.velocity.runtime.directive.Macro
directive.5=org.apache.velocity.runtime.directive.Literal
directive.6=org.apache.velocity.runtime.directive.Evaluate
directive.7=org.apache.velocity.runtime.directive.Break
directive.8=org.apache.velocity.runtime.directive.Define
directive.9=org.apache.velocity.runtime.directive.Stop