FreeMarker在JAVA中应用入门

在项目中通常有生成XML文件发送到另一个系统的需求,简单的办法可以是用一个XML模板,通过Freemarker替换其中的'Mark'(${}),生成最终的XML文件.

下面记录了一下简单的示例步骤:

1,创建一个XML模板:

<?xml version="1.0" encoding="UTF-8"?><people  xmlns:h="http://www.w3.org/TR/html4/">    <person id="000001" age="20">        <name>            <family>${p.fname}</family>            <given>${p.gname}</given>        </name>        <email>${p.email}</email>        <link manager="${p.manager}" />         </person>    </people>

2,在JAVA中将mark用值替换掉:下面是JAVA代码

package com.test.xml.freemarker;public class ValueObject {    private String fname;    private String gname;    private String email;    private String manager;    public String getFname() {        return fname;    }    public void setFname(String fname) {        this.fname = fname;    }    public String getGname() {        return gname;    }    public void setGname(String gname) {        this.gname = gname;    }    public String getEmail() {        return email;    }    public void setEmail(String email) {        this.email = email;    }    public String getManager() {        return manager;    }    public void setManager(String manager) {        this.manager = manager;    }    }public class Test {    Configuration freeMarkerCfg = new Configuration();    Template template = null;    public Test() {        freeMarkerCfg.setClassForTemplateLoading(getClass(),"");        freeMarkerCfg.setObjectWrapper(new DefaultObjectWrapper());        try {            template = freeMarkerCfg.getTemplate("test.xml");        } catch (IOException e) {            // TODO            e.printStackTrace();        }    }    public void generateRequest(ValueObject obj) {        String reqFileName = "D:\\temp\\test.xml";        try {            Map<String, Object> parameters = new HashMap<String, Object>();            parameters.put("p", obj);            OutputStreamWriter writer = new OutputStreamWriter(new FileOutputStream(reqFileName), "UTF-8");            template.process(parameters, writer);            writer.flush();        } catch (IOException e) {            e.printStackTrace();// TODO        } catch (TemplateException e) {            e.printStackTrace();// TODO        }    }    public static void main(String[] args) {        ValueObject val = new ValueObject();                val.setFname("Yao");        val.setGname("Yorker");        val.setEmail("[e-mail]");        val.setManager("manager");                new Test().generateRequest(val);    }}

XML模板和class在package "com.test.xml.freemarker".

***在模板中通过${val}指定的值,如果在处理的时候val值为null,会出现下面的异常:

freemarker.core.InvalidReferenceException: Expression valis undefined on line 46, column 63 in test.ftl

FreeMarker这么做的本意是防止在传值的地方(Data Model)写入的参数,如想传入 parameters.put("abc", obj);,但是写成了parameters.put("abcd", obj);

但是有的时候,里面有的值不是必须有值的,可以通过${val!""}来绕过这个异常.${val!""}的意思是如果val为null,取值"".

***Freemarker对XML文件中特殊字符的处理:通过<#escape>

    <#escape x as x?xml>    <person id="000001" age="20">        <name>            <family>${p.fname}</family>            <given>${p.gname}</given>        </name>        <email>${p.email}</email>        <link manager="${p.manager}" />    </person>    </#escape>

val.setManager("m&an<ager"); 传入的值就按照XML规范保存到XML文件 link manager="m&amp;an&lt;ager" />

***循环处理 <#list>

    <#list people as p>    <person id="000001" age="20">        <name>            <family>${p.fname}</family>            <given>${p.gname}</given>        </name>        <email>${p.email}</email>        <link manager="${p.manager}" />    </person>    </#list>        List<ValueObject> pList = new ArrayList<ValueObject>();        ValueObject val = null;                val = new ValueObject();                val.setFname("Yao");        val.setGname("Yorker");        val.setEmail("[e-mail]");        val.setManager("m&an<ager");                pList.add(val);                val = new ValueObject();                val.setFname("J");        val.setGname("Jeremy");        val.setEmail("[e-mail]");        val.setManager("m&an<ager");                pList.add(val);        parameters.put("people", pList);

***分支处理<#if>,根据值对模板做不同的输出.

        <#if p.level == "L1">
        <l1tag>xxx</l1tag>
        </#if>

***对生成超大文件的测试:

    public void generateRequest(List<ValueObject> pList) {        String reqFileName = "D:\\temp\\test.xml";        try {            Map<String, Object> parameters = new HashMap<String, Object>();            parameters.put("people", pList);            OutputStreamWriter writer = new OutputStreamWriter(new FileOutputStream(reqFileName), "UTF-8");            template.process(parameters, writer);            writer.flush();        } catch (IOException e) {            e.printStackTrace();// TODO        } catch (TemplateException e) {            e.printStackTrace();// TODO        }    }    public static void testLargeVolumn(){        List<ValueObject> pList = new ArrayList<ValueObject>();        ValueObject val = null;                for(int i=0;i<400000;i++){            val = new ValueObject();                    val.setFname("Yao"+i);            val.setGname("Yorker");            val.setEmail("[e-mail]");            val.setManager("m&an<ager");            val.setLevel("L"+i);            pList.add(val);        }                new Test().generateRequest(pList);    }

一次性输出,本例在达到循环400000次的时候异常:java.lang.OutOfMemoryError: Java heap space

解决办法:分多次输出到最终的输出文件中.

1,将模板需要循环的部分单独做成一个模板.

先一次输出循环模板前面的部分,分多次输出循环部分,一次输出循环部分之后的部分.

示例如下,(这里由于循环之前部分和之后部分不涉及模板替换,直接用JAVA代码输出)

模板需要循环输出的部分:

    <#escape x as x?xml>    <#list people as p>    <person id="000001" age="20">        <name>            <family>${p.fname}</family>            <given>${p.gname}</given>        </name>        <email>${p.email}</email>        <link manager="${p.manager}" />        <#if p.level == "L1">        <l1tag>xxx</l1tag>        </#if>    </person>    </#list>    </#escape>

JAVA代码:

public class LargeVolumnTest {    Configuration freeMarkerCfg = new Configuration();    Template template = null;    public LargeVolumnTest() {        freeMarkerCfg.setClassForTemplateLoading(getClass(),"");        freeMarkerCfg.setObjectWrapper(new DefaultObjectWrapper());        try {            template = freeMarkerCfg.getTemplate("testL.xml");        } catch (IOException e) {            // TODO            e.printStackTrace();        }    }    public void generateRequestHeader() {        String reqFileName = "D:\\temp\\test.xml";        try {            OutputStreamWriter writer = new OutputStreamWriter(new FileOutputStream(reqFileName), "UTF-8");            writer.write("<?xml version=\"1.0\" encoding=\"UTF-8\"?>");            writer.write("<people  xmlns:h=\"http://www.w3.org/TR/html4/\">");            writer.flush();        } catch (IOException e) {            e.printStackTrace();// TODO        }     }    public void generateRequestTail() {        String reqFileName = "D:\\temp\\test.xml";        try {            OutputStreamWriter writer = new OutputStreamWriter(new FileOutputStream(reqFileName,true), "UTF-8");            writer.write("</people>");                        writer.flush();        } catch (IOException e) {            e.printStackTrace();// TODO        }     }        public void generateRequest(List<ValueObject> pList) {        String reqFileName = "D:\\temp\\test.xml";        try {            Map<String, Object> parameters = new HashMap<String, Object>();            parameters.put("people", pList);            OutputStreamWriter writer = new OutputStreamWriter(new FileOutputStream(reqFileName,true), "UTF-8");            template.process(parameters, writer);            writer.flush();        } catch (IOException e) {            e.printStackTrace();// TODO        } catch (TemplateException e) {            e.printStackTrace();// TODO        }    }    public  void testLargeVolumn(){        generateRequestHeader();        List<ValueObject> pList = new ArrayList<ValueObject>();        ValueObject val = null;        int i=0;        for(;i<500000;i++){            val = new ValueObject();                    val.setFname("Yao"+i);            val.setGname("Yorker");            val.setEmail("[e-mail]");            val.setManager("m&an<ager");            val.setLevel("L"+i);            pList.add(val);            if(i%10000==0){                generateRequest(pList);                pList.clear();            }        }        if(i%10000!=0){            generateRequest(pList);        }        generateRequestTail();    }    public static void main(String[] args) {                new LargeVolumnTest().testLargeVolumn();    }}

注意在输出循环部分和结尾部分是通过追加到文件的方式:OutputStreamWriter writer = new OutputStreamWriter(new FileOutputStream(reqFileName,true), "UTF-8");

最终生成的XML文档有90M,没有报异常.

这里有FreeMarker的中文manual :http://download.csdn.net/user/kkdelta