FreeMarker在JAVA中應用入門

在項目中通常有生成XML文件發送到另一個系統的需求,簡單的辦法可以是用一個XML模板,通過Freemarker替換其中的'Mark'(${}),生成最終的XML文件.

下面記錄了一下簡單的示例步驟:

1,創建一個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中將mark用值替換掉:下面是JAVA代碼

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".

***在模板中通過${val}指定的值,如果在處理的時候val值為null,會出現下面的異常:

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

FreeMarker這么做的本意是防止在傳值的地方(Data Model)寫入的參數,如想傳入 parameters.put("abc", obj);,但是寫成了parameters.put("abcd", obj);

但是有的時候,里面有的值不是必須有值的,可以通過${val!""}來繞過這個異常.${val!""}的意思是如果val為null,取值"".

***Freemarker對XML文件中特殊字符的處理:通過<#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"); 傳入的值就按照XML規范保存到XML文件 link manager="m&amp;an&lt;ager" />

***循環處理 <#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);

***分支處理<#if>,根據值對模板做不同的輸出.

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

***對生成超大文件的測試:

    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);
    }

一次性輸出,本例在達到循環400000次的時候異常:java.lang.OutOfMemoryError: Java heap space

解決辦法:分多次輸出到最終的輸出文件中.

1,將模板需要循環的部分單獨做成一個模板.

先一次輸出循環模板前面的部分,分多次輸出循環部分,一次輸出循環部分之后的部分.

示例如下,(這里由於循環之前部分和之后部分不涉及模板替換,直接用JAVA代碼輸出)

模板需要循環輸出的部分:

    <#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代碼:

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();
    }
}

注意在輸出循環部分和結尾部分是通過追加到文件的方式:OutputStreamWriter writer = new OutputStreamWriter(new FileOutputStream(reqFileName,true), "UTF-8");

最終生成的XML文檔有90M,沒有報異常.

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