设计模式的应用场景(17)--职责链模式

职责链模式

定义：使多个对象都有机会处理请求，从而避免请求的发送者和接收者之间的耦合关系。将这些对象连成一条链，并沿着这条链传递该请求直到有一个处理它为止。

优点：降低类之间的耦合度，使得处理类仅需要保持一个指向后继者的引用，也使得一个对象无需知道是其他哪一个对象处理其请求，对象仅需知道该请求会被正确处理。接收者和发送者都没有明确的对方信息，增强了给对象指派职责的灵活性。

缺点：因为对象不知道是其它哪一个对象处理其请求的，所以职责链模式不保证对象被接收。

使用时机：当有多个对象处理请求，而且需要动态指定处理者的时候，可以使用职责链模式。

例子：
某公司的内部财务支出需要网上审批，对于不同额度的需要不同级别的人审批。这个可以用职责链模式实现。

首先设计一个抽象类，代表审批的人

public abstract class Approver {    public abstract void approve(BorrowBill borrowBill);    public Approver getSuccessor(){ return successor; }    public void setSuccessor(Approver successor){ this.successor = successor; }    private Approver successor;}

后面实现具体的各级别审批者，涉及金额不一样

public class DepartApprover extends Approver {    public void approve(BorrowBill borrowBill) {        if (borrowBill.getAmt() < 5000d) {            System.out.println("该员工部门主管审批了该借支单，借支单名称为：" + borrowBill.getName() + ",该借支单金额为：" + borrowBill.getAmt());        } else if (super.getSuccessor() != null) {            super.getSuccessor().approve(borrowBill);        }    }}public class FinanceApprover extends Approver {    public void approve(BorrowBill borrowBill) {        if (borrowBill.getAmt() >= 5000d && borrowBill.getAmt() < 10000d) {            System.out.println("财务部门主管审批了该借支单，借支单名称为：" + borrowBill.getName() + ",该借支单金额为：" + borrowBill.getAmt());        } else if (super.getSuccessor() != null) {            super.getSuccessor().approve(borrowBill);        }    }}public class VicepresidentApprover extends Approver {    public void approve(BorrowBill borrowBill) {        if (borrowBill.getAmt() >= 10000d && borrowBill.getAmt() < 20000d) {            System.out.println("财务副总审批了该借支单，借支单名称为：" + borrowBill.getName() + ",该借支单金额为：" + borrowBill.getAmt());        } else if (super.getSuccessor() != null) {            super.getSuccessor().approve(borrowBill);        }    }}public class ManagerApprover extends Approver {    public void approve(BorrowBill borrowBill) {        if (borrowBill.getAmt() >= 20000d && borrowBill.getAmt() < 50000d) {            System.out.println("公司总经理审批了该借支单，借支单名称为：" + borrowBill.getName() + ",该借支单金额为：" + borrowBill.getAmt());        } else if (super.getSuccessor() != null) {            super.getSuccessor().approve(borrowBill);        }    }}public class ChairmanApprover extends Approver {    public void approve(BorrowBill borrowBill) {        if (borrowBill.getAmt() >= 50000d) {            System.out.println("公司董事长审批了该借支单，借支单名称为：" + borrowBill.getName() + ",该借支单金额为：" + borrowBill.getAmt());        } else if (super.getSuccessor() != null) {            System.out.println("公司董事长没有上级了");        }    }}

最后客户端把审批者都加进来，形成职责链

public class Client {    public static void main(String[] argv) {        Approver departApprover = new DepartApprover();        Approver financeApprover = new FinanceApprover();        Approver vicepresidentApprover = new VicepresidentApprover();        Approver managerApprover = new ManagerApprover();        Approver chairmanApprover = new ChairmanApprover();        departApprover.setSuccessor(financeApprover);        financeApprover.setSuccessor(vicepresidentApprover);        vicepresidentApprover.setSuccessor(managerApprover);        managerApprover.setSuccessor(chairmanApprover);        BorrowBill borrowBill = new BorrowBill();        borrowBill.setName("出差借支单");        borrowBill.setAmt(3000d);        departApprover.approve(borrowBill);        borrowBill.setName("出差借支单");        borrowBill.setAmt(6000d);        departApprover.approve(borrowBill);        borrowBill.setName("采购办公用品借支单");        borrowBill.setAmt(15000d);        departApprover.approve(borrowBill);        borrowBill.setName("采购原料借支单");        borrowBill.setAmt(45000d);        departApprover.approve(borrowBill);        borrowBill.setName("采购原料借支单");        borrowBill.setAmt(100000d);        departApprover.approve(borrowBill);    }}