构建者（Builder）设计模式（又叫生成器设计模式）：
当一个类的内部数据过于复杂的时候（通常是负责持有数据的类，比如Config、VO、PO、Entity...），要创建的话可能就需要了解这个类的内部结构，还有这些东西是怎么组织装配等一大坨乱七八糟的东西，这个时候就会增加学习成本而且会很混乱，这个时候就想啊想一种什么法子来管理一下这个类中的数据呢，怎么在创建的时候让它按部就班的来，并且代码可读性很好别让我看花了眼啊，我要的东西也能都很好设置进来，这就是Builder模式的应用场景，Builder模式可以将一个类的构建和表示进行分离。
看一个例子：
public class Student {
private int id;
private String name;
private String passwd;
private String sex;
private String address;
// 构造器尽量缩小范围
private Student() {
}
// 构造器尽量缩小范围
private Student(Student origin) {
// 拷贝一份
this.id = origin.id;
this.name = origin.name;
this.passwd = origin.passwd;
this.sex = origin.sex;
this.address = origin.address;
}
public int getId() {
return id;
}
public String getName() {
return name;
}
public String getPasswd() {
return passwd;
}
public String getSex() {
return sex;
}
public String getAddress() {
return address;
}
/**
* Student的创建完全依靠Student.Builder，使用一种方法链的方式来创建
*
*/
public static class Builder {
private Student target;
public Builder() {
target = new Student();
}
public Builder address(int id) {
target.id = id;
return this;
}
public Builder name(String name) {
target.name = name;
return this;
}
public Builder password(String passwd) {
target.passwd = passwd;
return this;
}
public Builder sex(String sex) {
target.sex = sex;
return this;
}
public Builder address(String address) {
target.address = address;
return this;
}
public Student build() {
return new Student(target);
}
}
}
Student并不是直接new出来的，对其构造器进行了处理使其可访问范围尽可能的小，只让它通过Student.Builder来构建自己，在Student.Builder中提供了一种类set的方法链的方式来设置值，然后在最后的build()方法的时候会返回一个Student对象，现在要创建一个Student对象，代码如下：
Student s=new Student.Builder().name("CC").password("qwerty").sex("男").address("银河系第二旋臂").build();
再对比一下如果不使用构造者模式（一般情况下的用法）：
/**
* 学生实体
* @author CC11001100
*
*/
public class Student {
private int id;
private String name;
private String passwd;
private String sex;
private String address;
public Student() {
}
public Student(String name, String passwd, String sex, String address) {
super();
this.name = name;
this.passwd = passwd;
this.sex = sex;
this.address = address;
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getPasswd() {
return passwd;
}
public void setPasswd(String passwd) {
this.passwd = passwd;
}
public String getSex() {
return sex;
}
public void setSex(String sex) {
this.sex = sex;
}
public String getAddress() {
return address;
}
public void setAddress(String address) {
this.address = address;
}
}
创建对象：
Student s=new Student("CC","qwerty","男","银河系第二旋臂");
对比一下进行一个优劣性分析：
一般的套路：优点是比较简单，开发效率高，缺点是如果参数真的很多的话鬼知道每个对应的是什么意思啊。
Builder模式：优点是可以将构造器的setter方法名取成类似注释的方式，这样我们可以很清晰的知道刚才究竟设置的什么值，可读性较高，缺点是比较冗长。
总结：初步的理解Builder模式解决了要设置的参数过多不好管理的问题（感觉每次构建不同对象是废话 - -）。
从Struts2框架中拿出来的两个Builder模式的例子（都是Config类）：
ActionConfig ：
public class ActionConfig extends Located implements Serializable {
public static final String WILDCARD = "*";
protected List<InterceptorMapping> interceptors; // a list of interceptorMapping Objects eg. List<InterceptorMapping>
protected Map<String,String> params;
protected Map<String, ResultConfig> results;
protected List<ExceptionMappingConfig> exceptionMappings;
protected String className;
protected String methodName;
protected String packageName;
protected String name;
protected Set<String> allowedMethods;
protected ActionConfig(String packageName, String name, String className) {
this.packageName = packageName;
this.name = name;
this.className = className;
params = new LinkedHashMap<String, String>();
results = new LinkedHashMap<String, ResultConfig>();
interceptors = new ArrayList<InterceptorMapping>();
exceptionMappings = new ArrayList<ExceptionMappingConfig>();
allowedMethods = new HashSet<String>();
allowedMethods.add(WILDCARD);
}
/**
* Clones an ActionConfig, copying data into new maps and lists
* @param orig The ActionConfig to clone
* @Since 2.1
*/
protected ActionConfig(ActionConfig orig) {
this.name = orig.name;
this.className = orig.className;
this.methodName = orig.methodName;
this.packageName = orig.packageName;
this.params = new LinkedHashMap<String,String>(orig.params);
this.interceptors = new ArrayList<InterceptorMapping>(orig.interceptors);
this.results = new LinkedHashMap<String,ResultConfig>(orig.results);
this.exceptionMappings = new ArrayList<ExceptionMappingConfig>(orig.exceptionMappings);
this.allowedMethods = new HashSet<String>(orig.allowedMethods);
}
public String getName() {
return name;
}
public String getClassName() {
return className;
}
public List<ExceptionMappingConfig> getExceptionMappings() {
return exceptionMappings;
}
public List<InterceptorMapping> getInterceptors() {
return interceptors;
}
public Set<String> getAllowedMethods() {
return allowedMethods;
}
/**
* Returns name of the action method
*
* @return name of the method to execute
*/
public String getMethodName() {
return methodName;
}
/**
* @return Returns the packageName.
*/
public String getPackageName() {
return packageName;
}
public Map<String, String> getParams() {
return params;
}
public Map<String, ResultConfig> getResults() {
return results;
}
public boolean isAllowedMethod(String method) {
if (allowedMethods.size() == 1 && WILDCARD.equals(allowedMethods.iterator().next())) {
return true;
} else {
return allowedMethods.contains(method);
}
}
@Override public boolean equals(Object o) {
if (this == o) {
return true;
}
if (!(o instanceof ActionConfig)) {
return false;
}
final ActionConfig actionConfig = (ActionConfig) o;
if ((className != null) ? (!className.equals(actionConfig.className)) : (actionConfig.className != null)) {
return false;
}
if ((name != null) ? (!name.equals(actionConfig.name)) : (actionConfig.name != null)) {
return false;
}
if ((interceptors != null) ? (!interceptors.equals(actionConfig.interceptors)) : (actionConfig.interceptors != null))
{
return false;
}
if ((methodName != null) ? (!methodName.equals(actionConfig.methodName)) : (actionConfig.methodName != null)) {
return false;
}
if ((params != null) ? (!params.equals(actionConfig.params)) : (actionConfig.params != null)) {
return false;
}
if ((results != null) ? (!results.equals(actionConfig.results)) : (actionConfig.results != null)) {
return false;
}
if ((allowedMethods != null) ? (!allowedMethods.equals(actionConfig.allowedMethods)) : (actionConfig.allowedMethods != null)) {
return false;
}
return true;
}
@Override public int hashCode() {
int result;
result = (interceptors != null ? interceptors.hashCode() : 0);
result = 31 * result + (params != null ? params.hashCode() : 0);
result = 31 * result + (results != null ? results.hashCode() : 0);
result = 31 * result + (exceptionMappings != null ? exceptionMappings.hashCode() : 0);
result = 31 * result + (className != null ? className.hashCode() : 0);
result = 31 * result + (methodName != null ? methodName.hashCode() : 0);
result = 31 * result + (packageName != null ? packageName.hashCode() : 0);
result = 31 * result + (name != null ? name.hashCode() : 0);
result = 31 * result + (allowedMethods != null ? allowedMethods.hashCode() : 0);
return result;
}
@Override public String toString() {
StringBuilder sb = new StringBuilder();
sb.append("{ActionConfig ");
sb.append(name).append(" (");
sb.append(className);
if (methodName != null) {
sb.append(".").append(methodName).append("()");
}
sb.append(")");
sb.append(" - ").append(location);
sb.append("}");
return sb.toString();
}
/**
* The builder for this object. An instance of this object is the only way to construct a new instance. The
* purpose is to enforce the immutability of the object. The methods are structured in a way to support chaining.
* After setting any values you need, call the {@link #build()} method to create the object.
*/
public static class Builder implements InterceptorListHolder{
private ActionConfig target;
public Builder(ActionConfig toClone) {
target = new ActionConfig(toClone);
}
public Builder(String packageName, String name, String className) {
target = new ActionConfig(packageName, name, className);
}
public Builder packageName(String name) {
target.packageName = name;
return this;
}
public Builder name(String name) {
target.name = name;
return this;
}
public Builder className(String name) {
target.className = name;
return this;
}
public Builder defaultClassName(String name) {
if (StringUtils.isEmpty(target.className)) {
target.className = name;
}
return this;
}
public Builder methodName(String method) {
target.methodName = method;
return this;
}
public Builder addExceptionMapping(ExceptionMappingConfig exceptionMapping) {
target.exceptionMappings.add(exceptionMapping);
return this;
}
public Builder addExceptionMappings(Collection<? extends ExceptionMappingConfig> mappings) {
target.exceptionMappings.addAll(mappings);
return this;
}
public Builder exceptionMappings(Collection<? extends ExceptionMappingConfig> mappings) {
target.exceptionMappings.clear();
target.exceptionMappings.addAll(mappings);
return this;
}
public Builder addInterceptor(InterceptorMapping interceptor) {
target.interceptors.add(interceptor);
return this;
}
public Builder addInterceptors(List<InterceptorMapping> interceptors) {
target.interceptors.addAll(interceptors);
return this;
}
public Builder interceptors(List<InterceptorMapping> interceptors) {
target.interceptors.clear();
target.interceptors.addAll(interceptors);
return this;
}
public Builder addParam(String name, String value) {
target.params.put(name, value);
return this;
}
public Builder addParams(Map<String,String> params) {
target.params.putAll(params);
return this;
}
public Builder addResultConfig(ResultConfig resultConfig) {
target.results.put(resultConfig.getName(), resultConfig);
return this;
}
public Builder addResultConfigs(Collection<ResultConfig> configs) {
for (ResultConfig rc : configs) {
target.results.put(rc.getName(), rc);
}
return this;
}
public Builder addResultConfigs(Map<String,ResultConfig> configs) {
target.results.putAll(configs);
return this;
}
public Builder addAllowedMethod(String methodName) {
target.allowedMethods.add(methodName);
return this;
}
public Builder addAllowedMethod(Collection<String> methods) {
target.allowedMethods.addAll(methods);
return this;
}
public Builder location(Location loc) {
target.location = loc;
return this;
}
public ActionConfig build() {
target.params = Collections.unmodifiableMap(target.params);
target.results = Collections.unmodifiableMap(target.results);
target.interceptors = Collections.unmodifiableList(target.interceptors);
target.exceptionMappings = Collections.unmodifiableList(target.exceptionMappings);
target.allowedMethods = Collections.unmodifiableSet(target.allowedMethods);
ActionConfig result = target;
target = new ActionConfig(target);
return result;
}
}
}
ExceptionMappingConfig：
public class ExceptionMappingConfig extends Located implements Serializable {
private String name;
private String exceptionClassName;
private String result;
private Map<String,String> params;
protected ExceptionMappingConfig(String name, String exceptionClassName, String result) {
this.name = name;
this.exceptionClassName = exceptionClassName;
this.result = result;
this.params = new LinkedHashMap<String,String>();
}
protected ExceptionMappingConfig(ExceptionMappingConfig target) {
this.name = target.name;
this.exceptionClassName = target.exceptionClassName;
this.result = target.result;
this.params = new LinkedHashMap<String,String>(target.params);
}
public String getName() {
return name;
}
public String getExceptionClassName() {
return exceptionClassName;
}
public String getResult() {
return result;
}
public Map<String,String> getParams() {
return params;
}
@Override
public boolean equals(Object o) {
if (this == o) {
return true;
}
if (!(o instanceof ExceptionMappingConfig)) {
return false;
}
final ExceptionMappingConfig exceptionMappingConfig = (ExceptionMappingConfig) o;
if ((name != null) ? (!name.equals(exceptionMappingConfig.name)) : (exceptionMappingConfig.name != null)) {
return false;
}
if ((exceptionClassName != null) ? (!exceptionClassName.equals(exceptionMappingConfig.exceptionClassName)) : (exceptionMappingConfig.exceptionClassName != null))
{
return false;
}
if ((result != null) ? (!result.equals(exceptionMappingConfig.result)) : (exceptionMappingConfig.result != null))
{
return false;
}
if ((params != null) ? (!params.equals(exceptionMappingConfig.params)) : (exceptionMappingConfig.params != null))
{
return false;
}
return true;
}
@Override
public int hashCode() {
int hashCode;
hashCode = ((name != null) ? name.hashCode() : 0);
hashCode = (29 * hashCode) + ((exceptionClassName != null) ? exceptionClassName.hashCode() : 0);
hashCode = (29 * hashCode) + ((result != null) ? result.hashCode() : 0);
hashCode = (29 * hashCode) + ((params != null) ? params.hashCode() : 0);
return hashCode;
}
/**
* The builder for this object. An instance of this object is the only way to construct a new instance. The
* purpose is to enforce the immutability of the object. The methods are structured in a way to support chaining.
* After setting any values you need, call the {@link #build()} method to create the object.
*/
public static class Builder{
private ExceptionMappingConfig target;
public Builder(ExceptionMappingConfig toClone) {
target = new ExceptionMappingConfig(toClone);
}
public Builder(String name, String exceptionClassName, String result) {
target = new ExceptionMappingConfig(name, exceptionClassName, result);
}
public Builder name(String name) {
target.name = name;
return this;
}
public Builder exceptionClassName(String name) {
target.exceptionClassName = name;
return this;
}
public Builder result(String result) {
target.result = result;
return this;
}
public Builder addParam(String name, String value) {
target.params.put(name, value);
return this;
}
public Builder addParams(Map<String,String> params) {
target.params.putAll(params);
return this;
}
public Builder location(Location loc) {
target.location = loc;
return this;
}
public ExceptionMappingConfig build() {
target.params = Collections.unmodifiableMap(target.params);
ExceptionMappingConfig result = target;
target = new ExceptionMappingConfig(target);
return result;
}
}
}
参考资料：
