Android Studio中用JUnit和Mockito测试android.icu.text相关Java类的问题

Android单元测试环境中，android.icu.text下的类属于Android框架API，默认JUnit单元测试环境无法真实实例化这些类。开启unitTests.returnDefaultValues = true后，框架会返回默认值（如null），导致DecimalFormat.format()返回null，测试失败。

方案1：用Mockito模拟依赖（推荐） #

通过Mockito模拟DecimalFormat和DecimalFormatSymbols的行为，避免直接依赖Android框架类。先添加Mockito依赖到build.gradle：

dependencies {
    testImplementation 'org.mockito:mockito-core:4.11.0'
    testImplementation 'junit:junit:4.13.2'
}

修改测试类，模拟方法逻辑：

import android.icu.text.DecimalFormat;
import android.icu.text.DecimalFormatSymbols;
import org.junit.Assert;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.mockito.Mock;
import org.mockito.Mockito;
import org.mockito.junit.MockitoJUnitRunner;

@RunWith(MockitoJUnitRunner.class)
public class NumberFormatterTest {

    @Test
    public void format_givenInputStringAndPatternString_shouldReturnCorrespondingFormattedString(){
        String pattern = "#,###.##";
        String inputString = "1000.12";
        String expect = "1,000.12";

        NumberFormatter formatter = Mockito.spy(new NumberFormatter());
        DecimalFormat mockFormat = Mockito.mock(DecimalFormat.class);
        Mockito.when(mockFormat.format(1000.12d)).thenReturn(expect);
        Mockito.doReturn(mockFormat).when(formatter).getDecimalFormat(Mockito.anyString(), Mockito.any(DecimalFormatSymbols.class));

        String actual = formatter.format(inputString, pattern);
        Assert.assertEquals(expect, actual);
    }
}

方案2：使用Robolectric测试 #

Robolectric可以模拟完整Android运行环境，支持android.icu类的真实行为。添加Robolectric依赖：

dependencies {
    testImplementation 'org.robolectric:robolectric:4.12.1'
    testImplementation 'junit:junit:4.13.2'
}

修改测试类，使用RobolectricRunner：

import org.junit.Assert;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.robolectric.RobolectricTestRunner;

@RunWith(RobolectricTestRunner.class)
public class NumberFormatterTest {

    @Test
    public void format_givenInputStringAndPatternString_shouldReturnCorrespondingFormattedString(){
        String pattern = "#,###.##";
        String inputString = "1000.12";
        String expect = "1,000.12";
        String actual = new NumberFormatter().format(inputString, pattern);

        Assert.assertEquals(expect, actual);
    }
}

方案3：重构代码，依赖抽象 #

将格式化逻辑抽象成接口，测试时用Java标准库实现解耦：

1. 定义抽象接口 #

public interface NumberFormatStrategy {
    String format(double value, String pattern);
}

2. 实现Android端策略 #

import android.icu.text.DecimalFormat;
import android.icu.text.DecimalFormatSymbols;
import java.util.Locale;

public class AndroidIcuNumberFormatStrategy implements NumberFormatStrategy {
    @Override
    public String format(double value, String pattern) {
        DecimalFormatSymbols symbols = new DecimalFormatSymbols(Locale.getDefault());
        symbols.setDecimalSeparator('.');
        symbols.setGroupingSeparator(',');
        return new DecimalFormat(pattern, symbols).format(value);
    }
}

3. 修改NumberFormatter类 #

import androidx.annotation.NonNull;

public class NumberFormatter {
    private final NumberFormatStrategy formatStrategy;

    public NumberFormatter() {
        this(new AndroidIcuNumberFormatStrategy());
    }

    public NumberFormatter(@NonNull NumberFormatStrategy formatStrategy) {
        this.formatStrategy = formatStrategy;
    }

    public String format(String inputText, String pattern){
        double value = Double.parseDouble(inputText);
        return formatStrategy.format(value, pattern);
    }
}

4. 测试用Java标准库实现 #

import java.text.DecimalFormat;
import java.text.DecimalFormatSymbols;
import java.util.Locale;
import org.junit.Assert;
import org.junit.Test;

public class NumberFormatterTest {

    @Test
    public void format_givenInputStringAndPatternString_shouldReturnCorrespondingFormattedString(){
        String pattern = "#,###.##";
        String inputString = "1000.12";
        String expect = "1,000.12";

        NumberFormatStrategy testStrategy = (value, pat) -> {
            DecimalFormatSymbols symbols = new DecimalFormatSymbols(Locale.getDefault());
            symbols.setDecimalSeparator('.');
            symbols.setGroupingSeparator(',');
            return new DecimalFormat(pat, symbols).format(value);
        };

        String actual = new NumberFormatter(testStrategy).format(inputString, pattern);
        Assert.assertEquals(expect, actual);
    }
}

• unitTests.returnDefaultValues = true仅能避免测试崩溃，不会提供真实方法实现，因此会返回null。
• Mockito适合隔离测试业务逻辑，Robolectric适合模拟真实Android环境，重构抽象是最彻底的解耦方案。

内容的提问来源于stack exchange，提问作者Trần Võ Bảo