#329 Added declarative equivalent of JUnit's assertThrows

Author: peterdemaeyer

hamcrest/src/main/java/org/hamcrest/Executable.java

@@ -0,0 +1,7 @@
+package org.hamcrest;
+
+@FunctionalInterface
+public interface Executable {
+
+    void execute() throws Throwable;
+}

hamcrest/src/main/java/org/hamcrest/MatcherAssert.java

@@ -20,10 +20,37 @@ public static <T> void assertThat(String reason, T actual, Matcher<? super T> ma
             throw new AssertionError(description.toString());
         }
     }
-    
+
     public static void assertThat(String reason, boolean assertion) {
         if (!assertion) {
             throw new AssertionError(reason);
         }
     }
+
+    public static <T extends Throwable> void assertThat(Executable executable, Throws<T> doesThrow) {
+        assertThat("", executable, doesThrow);
+    }
+
+    public static <T extends Throwable> void assertThat(String reason, Executable executable, Throws<T> doesThrow) {
+        boolean executionDidNotThrow = false;
+        try {
+            executable.execute();
+            executionDidNotThrow = true;
+        } catch (Throwable actual) {
+            assertThat(reason, (T) actual, doesThrow.asMatcher());
+        } finally {
+            if (executionDidNotThrow) {
+                Description description = new StringDescription();
+                description.appendText(reason)
+                        .appendText(System.lineSeparator())
+                        .appendText("Expected: ")
+                        .appendDescriptionOf(doesThrow)
+                        .appendText(System.lineSeparator())
+                        .appendText("     but: ");
+                doesThrow.describeMismatch(description);
+
+                throw new AssertionError(description.toString());
+            }
+        }
+    }
 }

hamcrest/src/main/java/org/hamcrest/Matchers.java

@@ -1712,5 +1712,71 @@ public static org.hamcrest.Matcher<org.w3c.dom.Node> hasXPath(java.lang.String x
     return org.hamcrest.xml.HasXPath.hasXPath(xPath, namespaceContext);
   }
 
+  /**
+   * Creates a {@link Throws} object that matches a throwable according to the given throwable matcher.
+   * This can be used with the {@link MatcherAssert#assertThat(String, Executable, Throws)} family of methods.
+   * For example:
+   * <pre>assertThat(() -> methodCallThatThrowsException(), doesThrow(withMessage("file not found")))</pre>
+   *
+   * @param throwableMatcher
+   *     the matcher for the throwable to match, which must not be {@code null}
+   */
+  public static <T extends Throwable> Throws<T> doesThrow(Matcher<? super Throwable> throwableMatcher) {
+    return Throws.doesThrow(throwableMatcher);
+  }
+
+  /**
+   * Creates a {@link Throws} object that matches a throwable of the given type.
+   * This can be used with the {@link MatcherAssert#assertThat(String, Executable, Throws)} family of methods.
+   * For example:
+   * <pre>assertThat(() -> methodCallThatThrowsIOException(), throwsInstanceOf(IOException.class))</pre>
+   * This is shorthand for {@code doesThrow(instanceOf(MyThrowable.class))}, to be used as equivalent for JUnit 5's
+   * {@code assertThrows(MyThrowable.class, () -> {})}.
+   *
+   * @param throwableType
+   *     the type of the throwable to match, which must not be {@code null}
+   */
+  public static <T extends Throwable> Throws<T> throwsInstanceOf(Class<T> throwableType) {
+    return Throws.throwsInstanceOf(throwableType);
+  }
+
+  /**
+   * Creates a matcher that matches a throwable by matching its message.
+   * This can be used with the {@link MatcherAssert#assertThat(String, Executable, Throws)} family of methods.
+   * For example:
+   * <pre>assertThat(() -> methodCallThatThrowsException(), doesThrow(withMessage(startsWith("file not found"))))</pre>
+   *
+   * @param messageMatcher
+   *     the matcher to match the throwable's message with, which must not be {@code null}
+   */
+  public static <T extends Throwable> Matcher<T> withMessage(Matcher<String> messageMatcher) {
+    return Throws.withMessage(messageMatcher);
+  }
+
+  /**
+   * Creates a matcher that matches a throwable by its message.
+   * This can be used with the {@link MatcherAssert#assertThat(String, Executable, Throws)} family of methods.
+   * For example:
+   * <pre>assertThat(() -> methodCallThatThrowsException(), doesThrow(withMessage("message")))</pre>
+   * This is shorthand for {@code doesThrow(withMessage(equalTo("message")))}.
+   *
+   * @param messageToMatch
+   *     the message to match the throwable's message with, which must not be {@code null}
+   */
+  public static <T extends Throwable> Matcher<T> withMessage(String messageToMatch) {
+    return withMessage(equalTo(messageToMatch));
+  }
 
+  /**
+   * Creates a matcher that matches an outer throwable by matching its inner cause.
+   * This can be used with the {@link MatcherAssert#assertThat(String, Executable, Throws)} family of methods.
+   * For example:
+   * <pre>assertThat(() -> methodCallThatThrowsInvocationTargetException(), doesThrow(becauseOf(instanceOf(IOException.class))))</pre>
+   *
+   * @param causeMatcher
+   *     the matcher to matcher the outer throwable's inner cause with, which must not be {@code null}
+   */
+  public static <T extends Throwable> Matcher<T> becauseOf(Matcher<? extends Throwable> causeMatcher) {
+    return Throws.becauseOf(causeMatcher);
+  }
 }

hamcrest/src/main/java/org/hamcrest/Throws.java

@@ -0,0 +1,100 @@
+package org.hamcrest;
+
+import static java.util.Objects.requireNonNull;
+import static org.hamcrest.core.IsInstanceOf.instanceOf;
+
+/**
+ * @author Peter De Maeyer
+ */
+public final class Throws<T extends Throwable> implements SelfDescribing {
+
+    private final Matcher<? super T> matcher;
+
+    public Throws(Matcher<? super T> throwableMatcher) {
+        requireNonNull(throwableMatcher);
+        this.matcher = new BaseMatcher<T>() {
+
+            @Override
+            public boolean matches(Object actual) {
+                return throwableMatcher.matches(actual);
+            }
+
+            @Override
+            public void describeTo(Description description) {
+                description.appendText("throws ");
+                throwableMatcher.describeTo(description);
+            }
+
+            @Override
+            public void describeMismatch(Object item, Description mismatchDescription) {
+                mismatchDescription.appendText("threw but ");
+                throwableMatcher.describeMismatch(item, mismatchDescription);
+            }
+        };
+    }
+
+    Matcher<? super T> asMatcher() {
+        return matcher;
+    }
+
+    @Override
+    public void describeTo(Description description) {
+        matcher.describeTo(description);
+    }
+
+    public void describeMismatch(Description description) {
+        description.appendText("did not throw");
+    }
+
+    public static <T extends Throwable> Matcher<T> withMessage(Matcher<String> messageMatcher) {
+        return new TypeSafeMatcher<T>() {
+
+            @Override
+            protected boolean matchesSafely(T throwable) {
+                return messageMatcher.matches(throwable.getMessage());
+            }
+
+            @Override
+            public void describeTo(Description description) {
+                description.appendText("with message ");
+                messageMatcher.describeTo(description);
+            }
+
+            @Override
+            protected void describeMismatchSafely(T item, Description mismatchDescription) {
+                mismatchDescription.appendText("message ");
+                messageMatcher.describeMismatch(item.getMessage(), mismatchDescription);
+            }
+        };
+    }
+
+    public static <T extends Throwable> Matcher<T> becauseOf(Matcher<? extends Throwable> causeMatcher) {
+        return new TypeSafeMatcher<T>() {
+
+            @Override
+            protected boolean matchesSafely(T throwable) {
+                return causeMatcher.matches(throwable.getCause());
+            }
+
+            @Override
+            public void describeTo(Description description) {
+                description.appendText("because of ");
+                causeMatcher.describeTo(description);
+            }
+
+            @Override
+            protected void describeMismatchSafely(T item, Description mismatchDescription) {
+                mismatchDescription.appendText("cause ");
+                causeMatcher.describeMismatch(item.getCause(), mismatchDescription);
+            }
+        };
+    }
+
+    public static <T extends Throwable> Throws<T> doesThrow(Matcher<? super T> throwableMatcher) {
+        return new Throws<T>(throwableMatcher);
+    }
+
+    public static <T extends Throwable> Throws<T> throwsInstanceOf(Class<T> throwableType) {
+        return doesThrow(instanceOf(throwableType));
+    }
+}

hamcrest/src/test/java/org/hamcrest/MatcherAssertTest.java

@@ -1,9 +1,12 @@
 package org.hamcrest;
 
+import java.io.IOException;
+import javax.xml.stream.XMLStreamException;
+
 import org.junit.Test;
 
+import static org.hamcrest.Matchers.*;
 import static org.hamcrest.MatcherAssert.assertThat;
-import static org.hamcrest.core.IsEqual.equalTo;
 import static org.junit.Assert.*;
 
 public final class MatcherAssertTest {
@@ -96,4 +99,115 @@ public void describeMismatch(Object item, Description mismatchDescription) {
     canAssertSubtypes() {
         assertThat(1, equalTo((Number) 1));
     }
+
+    @Test public void
+    throwableIsOfMatchingInstance() {
+        assertThat(
+                () -> { throw new IllegalStateException(); },
+                throwsInstanceOf(IllegalStateException.class)
+        );
+    }
+
+    @Test public void
+    throwableIsNotOfMatchingInstance() {
+        String endLine = System.lineSeparator();
+        String expectedMessage = endLine + "Expected: throws an instance of java.io.IOException" + endLine
+                + "     but: threw but <java.lang.IllegalStateException> is a java.lang.IllegalStateException";
+        try {
+            assertThat(
+                    () -> { throw new IllegalStateException(); },
+                    throwsInstanceOf(IOException.class)
+            );
+            fail("should have failed");
+        }
+        catch (AssertionError e) {
+            assertEquals(expectedMessage, e.getMessage());
+        }
+    }
+
+    @Test public void
+    throwableHasMatchingMessage() {
+        assertThat(
+                () -> { throw new Exception("message"); },
+                doesThrow(withMessage(equalTo("message")))
+        );
+    }
+
+    @Test public void
+    throwableDoesNotHaveMatchingMessage() {
+        String endLine = System.lineSeparator();
+        String expectedMessage = endLine + "Expected: throws with message \"expected message\"" + endLine
+                + "     but: threw but message was \"actual message\"";
+        try {
+            assertThat(
+                    () -> { throw new Exception("actual message"); },
+                    doesThrow(withMessage("expected message"))
+            );
+            fail("should have failed");
+        }
+        catch (AssertionError e) {
+            assertEquals(expectedMessage, e.getMessage());
+        }
+    }
+
+    @Test public void
+    throwableExecutionDoesNotThrow() {
+        String endLine = System.lineSeparator();
+        String expectedMessage = endLine + "Expected: throws an instance of java.lang.NoSuchMethodError"
+                + endLine + "     but: did not throw";
+        try {
+            assertThat(
+                    () -> {}, // Do nothing
+                    throwsInstanceOf(NoSuchMethodError.class)
+            );
+            fail("should have failed");
+        }
+        catch (AssertionError e) {
+            assertEquals(expectedMessage, e.getMessage());
+        }
+    }
+
+    @Test public void
+    throwableCauseMatches() {
+        assertThat(
+                () -> { throw new RuntimeException(new XMLStreamException()); },
+                doesThrow(becauseOf(instanceOf(XMLStreamException.class)))
+        );
+    }
+
+    @Test public void
+    throwableCauseDoesNotMatch() {
+        String endLine = System.lineSeparator();
+        String expectedMessage = endLine + "Expected: throws because of an instance of java.lang.NullPointerException"
+                + endLine + "     but: threw but cause <java.lang.IllegalArgumentException> is a java.lang.IllegalArgumentException";
+        try {
+            assertThat(
+                    () -> { throw new RuntimeException(new IllegalArgumentException()); },
+                    doesThrow(becauseOf(instanceOf(NullPointerException.class)))
+            );
+            fail("should have failed");
+        }
+        catch (AssertionError e) {
+            assertEquals(expectedMessage, e.getMessage());
+        }
+    }
+
+    @Test public void
+    throwableExecutionDoesNotMatchWithCustomMessage() {
+        String endLine = System.lineSeparator();
+        String expectedMessage = "Custom message"
+                + endLine + "Expected: throws an instance of java.lang.NullPointerException"
+                + endLine + "     but: threw but <java.lang.IllegalArgumentException> is a java.lang.IllegalArgumentException";
+        try {
+            assertThat(
+                    "Custom message",
+                    () -> { throw new IllegalArgumentException(); },
+                    throwsInstanceOf(NullPointerException.class)
+            );
+            fail("should have failed");
+        }
+        catch (AssertionError e) {
+            assertEquals(expectedMessage, e.getMessage());
+        }
+    }
 }