java定义贷款类Loan，如果贷款总额、利率或年数小于等于0，抛出异常。

在Java中，你可以定义一个名为`Loan`的类，包含贷款总额（principal）、年利率（interestRate）和贷款年限（years）等属性，并添加适当的构造函数和getter/setter方法。为了确保输入的有效性，我们需要在设置这些属性值时添加验证。当贷款总额、利率或年数小于等于0时，我们将抛出自定义的异常或者使用内置的`IllegalArgumentException`。这里是一个简单的示例：

public class LoanException extends IllegalArgumentException {
    public LoanException(String message) {
        super(message);
    }
}

public class Loan {
    private double principal;
    private double interestRate;
    private int years;

    // 构造函数进行参数校验
    public Loan(double principal, double interestRate, int years) throws LoanException {
        if (principal <= 0 || interestRate <= 0 || years <= 0) {
            throw new LoanException("贷款金额、利率或年数不能小于等于0");
        }
        this.principal = principal;
        this.interestRate = interestRate;
        this.years = years;
    }

    // getter and setter methods
    public double getPrincipal() {
        return principal;
    }

    public void setPrincipal(double principal) {
        if (principal <= 0) {
            throw new IllegalArgumentException("贷款金额不能小于等于0");
        }
        this.principal = principal;
    }

    public double getInterestRate() {
        return interestRate;
    }

    public void setInterestRate(double interestRate) {
        if (interestRate <= 0) {
            throw new IllegalArgumentException("利率不能小于等于0");
        }
        this.interestRate = interestRate;
    }

    public int getYears() {
        return years;
    }

    public void setYears(int years) {
        if (years <= 0) {
            throw new IllegalArgumentException("年数不能小于等于0");
        }
        this.years = years;
    }
}

// 使用示例
try {
    Loan loan = new Loan(5000, 0.05, 5); // 正常创建贷款
} catch (LoanException e) {
    System.out.println(e.getMessage());
}

在这个例子中，如果你尝试创建一个无效的贷款对象，比如负本金或零利率，将会抛出`LoanException`。