Java 中的Dependency Injection 依赖注入

TaxReport.java

package com.company;

public class TaxReport {
    private TaxCalculator calculator;

    public TaxReport(TaxCalculator calculator){
        this.calculator = calculator;
    }

    public void show(){
    var tax = calculator.calculateTax();
        System.out.println(tax);
    }
}

把接口TaxCalculator直接写进来：

private TaxCalculator calculator;

    public TaxReport(TaxCalculator calculator){
        this.calculator = calculator;
    }

Main.java

package com.company;

public class Main {
    public static void main(String[] args) {
    var calculator = new TaxCalculator2018(100_000);
    var report = new TaxReport(calculator);
    }
}

var calculator = new TaxCalculator2018(100_000);

var report = new TaxReport(calculator); 

只有两个类，手动引入：POOR Man's dependency injection——需要Spring框架

TaxCalculator2018.java

package com.company;

public class TaxCalculator2018 implements TaxCalculator{
    private double taxableIncome;

    public TaxCalculator2018(double taxableIncome) {
        this.taxableIncome = taxableIncome;
    }

    @Override
    public double calculateTax(){
        return taxableIncome*0.3;
    }
}

可以改变类在运行时的依赖关系

TaxReport.java添加：

package com.company;

public class TaxReport {
    private TaxCalculator calculator;

    public TaxReport(TaxCalculator calculator){
        this.calculator = calculator;
    }

    public void show(){
    var tax = calculator.calculateTax();
        System.out.println(tax);
    }

    public void setCalculator(TaxCalculator calculator) {
        this.calculator = calculator;
    }
}

添加一个TaxCalculator2019.java

package com.company;

public class TaxCalculator2019 implements TaxCalculator{

    @Override
    public double calculateTax() {
        return 0;
    }
}

Main.java

package com.company;

public class Main {
    public static void main(String[] args) {
    var calculator = new TaxCalculator2018(100_000);
    var report = new TaxReport(calculator);
    report.show();

    report.setCalculator(new TaxCalculator2019());
    report.show();
    
}

3.Method Injection方法注入

TaxReport.java

public void show(TaxCalculator calculator){
var tax = calculator.calculateTax();
    System.out.println(tax);
}

 Main.java

package com.company;

public class Main {
    public static void main(String[] args) {
    var calculator = new TaxCalculator2018(100_000);
    var report = new TaxReport();
    report.show(calculator);

    report.show(new TaxCalculator2019());
    }
}

3种依赖注入方式！

Done!