Freemarker無法獲取對象屬性，報錯The following has evaluated to null or missing （實體類的屬性命名不規範導致）

最近寫freemarker項目的時候遇到一個錯The following has evaluated to null or missing，改了一上午才改好，特意寫下，以此爲鑑！
Music 實體類（問題就出在這！！！！！！！！）

import java.io.Serializable;
public class Music implements Serializable{
	private String M_Name;	 //歌名
	private String M_Singer; //歌手
	private String M_Time;   //時長
	private String M_Publish_Time;	 //發佈時間
	private String M_Album;   //專輯

	public Music() {
		super();
	}
	public Music(String mName, String mSinger, String mTime,
			String mPublishTime, String mAlbum) {
		super();
		M_Name = mName;
		M_Singer = mSinger;
		M_Time = mTime;
		M_Publish_Time = mPublishTime;
		M_Album = mAlbum;
	}
	//get、set方法略
}

ftl模板

${music.M_Name}
${music.M_Singer}
${music.M_Time}
${music.M_Publish_Time}
${music.M_Album}

測試類：

import java.io.File;
import java.io.FileWriter;
import java.io.Writer;
import java.util.HashMap;
import java.util.Map;

import freemarker.template.Configuration;
import freemarker.template.Template;


public class FreeMarkerTest {
	   public static void main(String[] args) throws Exception { 
	        String dir="C:\\Users\\xyk\\Workspaces\\MyEclipse 8.6\\StaticHtml\\WebRoot\\WEB-INF\\templates";
			Configuration conf = new Configuration();
			//加載模板文件(模板的路徑)在此用的是磁盤下的具體路徑
			conf.setDirectoryForTemplateLoading(new File(dir));
			// 加載模板
			Template template = conf.getTemplate("/test.ftl");
			// 定義數據	
			//多個個數據
			/*Map root = new HashMap();
			root.put("a", 123);
			root.put("b", 456);	*/
			
			//單個對象
			Map <String,Object> root  =  new HashMap<String,Object>();;
			Music music=new Music("陰天","莫文蔚","3:54","2013.10.23","莫後年代 莫文蔚20週年世紀典藏 (The Age of Moknificence)");
	       	       root.put("music", music);
	       	    
			// 定義輸出
			Writer out = new FileWriter(dir + "/test.html");
			template.process(root, out);
			System.out.println("轉換成功");
			out.flush();
			out.close();
	    } 
}

報錯：

嚴重: Error executing FreeMarker template
FreeMarker template error:
The following has evaluated to null or missing:
==> music.M_Name  [in template "test.ftl" at line 1, column 3]

----
Tip: It's the step after the last dot that caused this error, not those before it.
----
Tip: If the failing expression is known to be legally refer to something that's null or missing, either specify a default value like myOptionalVar!myDefault, or use <#if myOptionalVar??>when-present<#else>when-missing</#if>. (These only cover the last step of the expression; to cover the whole expression, use parenthesis: (myOptionalVar.foo)!myDefault, (myOptionalVar.foo)??
----

----
FTL stack trace ("~" means nesting-related):
	- Failed at: ${music.M_Name}  [in template "test.ftl" at line 1, column 1]
----

Java stack trace (for programmers):
----
freemarker.core.InvalidReferenceException: [... Exception message was already printed; see it above ...]
	at freemarker.core.InvalidReferenceException.getInstance(InvalidReferenceException.java:108)
	at freemarker.core.EvalUtil.coerceModelToString(EvalUtil.java:346)
	at freemarker.core.Expression.evalAndCoerceToString(Expression.java:80)
	at freemarker.core.DollarVariable.accept(DollarVariable.java:40)
	at freemarker.core.Environment.visit(Environment.java:257)
	at freemarker.core.MixedContent.accept(MixedContent.java:57)
	at freemarker.core.Environment.visit(Environment.java:257)
	at freemarker.core.Environment.process(Environment.java:235)
	at freemarker.template.Template.process(Template.java:262)
	at Test.FreeMarkerTest.main(FreeMarkerTest.java:40)
Exception in thread "main" FreeMarker template error:
The following has evaluated to null or missing:
==> music.M_Name  [in template "test.ftl" at line 1, column 3]

----
Tip: It's the step after the last dot that caused this error, not those before it.
----
Tip: If the failing expression is known to be legally refer to something that's null or missing, either specify a default value like myOptionalVar!myDefault, or use <#if myOptionalVar??>when-present<#else>when-missing</#if>. (These only cover the last step of the expression; to cover the whole expression, use parenthesis: (myOptionalVar.foo)!myDefault, (myOptionalVar.foo)??
----

----
FTL stack trace ("~" means nesting-related):
	- Failed at: ${music.M_Name}  [in template "test.ftl" at line 1, column 1]
----

Java stack trace (for programmers):
----
freemarker.core.InvalidReferenceException: [... Exception message was already printed; see it above ...]
	at freemarker.core.InvalidReferenceException.getInstance(InvalidReferenceException.java:108)
	at freemarker.core.EvalUtil.coerceModelToString(EvalUtil.java:346)
	at freemarker.core.Expression.evalAndCoerceToString(Expression.java:80)
	at freemarker.core.DollarVariable.accept(DollarVariable.java:40)
	at freemarker.core.Environment.visit(Environment.java:257)
	at freemarker.core.MixedContent.accept(MixedContent.java:57)
	at freemarker.core.Environment.visit(Environment.java:257)
	at freemarker.core.Environment.process(Environment.java:235)
	at freemarker.template.Template.process(Template.java:262)
	at Test.FreeMarkerTest.main(FreeMarkerTest.java:40)

注意：其實錯誤原因就是實體類的屬性命名不規範
private String M_Name; //歌名
private String M_Singer; //歌手
private String M_Time; //時長
private String M_Publish_Time; //發佈時間
private String M_Album; //專輯
屬性名字不符合javabean的規範！也就改 實體類 和 flt模板 ，把首字母改爲小寫！M–>m
改完後代碼：

import java.io.Serializable;
public class Music implements Serializable{
	private String m_Name;	 //歌名
	private String m_Singer; //歌手
	private String m_Time;   //時長
	private String m_Publish_Time;	 //發佈時間
	private String m_Album;   //專輯
	
	public Music() {
		super();
	}

	public Music(String mName, String mSinger, String mTime,
			String mPublishTime, String mAlbum) {
		super();
		m_Name = mName;
		m_Singer = mSinger;
		m_Time = mTime;
		m_Publish_Time = mPublishTime;
		m_Album = mAlbum;
	}
	//get、set方法略

}

ftl改後：

${music.m_Name}
${music.m_Singer}
${music.m_Time}
${music.m_Publish_Time}
${music.m_Album}

生成的html文件：