Throwables.java

package org.junit.internal;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.PrintWriter;
import java.io.StringReader;
import java.io.StringWriter;
import java.util.AbstractList;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.List;

/**
 * Miscellaneous functions dealing with {@code Throwable}.
 *
 * @author kcooney@google.com (Kevin Cooney)
 * @since 4.12
 */
public final class Throwables {

    private Throwables() {
    }

    /**
     * Rethrows the given {@code Throwable}, allowing the caller to
     * declare that it throws {@code Exception}. This is useful when
     * your callers have nothing reasonable they can do when a
     * {@code Throwable} is thrown. This is declared to return {@code Exception}
     * so it can be used in a {@code throw} clause:
     * <pre>
     * try {
     *   doSomething();
     * } catch (Throwable e} {
     *   throw Throwables.rethrowAsException(e);
     * }
     * doSomethingLater();
     * </pre>
     *
     * @param e exception to rethrow
     * @return does not return anything
     * @since 4.12
     */
    public static Exception rethrowAsException(Throwable e) throws Exception {
        Throwables.<Exception>rethrow(e);
        return null; // we never get here
    }

    @SuppressWarnings("unchecked")
    private static <T extends Throwable> void rethrow(Throwable e) throws T {
        throw (T) e;
    }

    /**
     * Returns the stacktrace of the given Throwable as a String.
     *
     * @since 4.13
     */
    public static String getStacktrace(Throwable exception) {
        StringWriter stringWriter = new StringWriter();
        PrintWriter writer = new PrintWriter(stringWriter);
        exception.printStackTrace(writer);
        return stringWriter.toString();
    }

    /**
     * Gets a trimmed version of the stack trace of the given exception. Stack trace
     * elements that are below the test method are filtered out.
     *
     * @return a trimmed stack trace, or the original trace if trimming wasn't possible
     */
    public static String getTrimmedStackTrace(Throwable exception) {
        List<String> trimmedStackTraceLines = getTrimmedStackTraceLines(exception);
        if (trimmedStackTraceLines.isEmpty()) {
            return getFullStackTrace(exception);
        }

        StringBuilder result = new StringBuilder(exception.toString());
        appendStackTraceLines(trimmedStackTraceLines, result);
        appendStackTraceLines(getCauseStackTraceLines(exception), result);
        return result.toString();
    }

    private static List<String> getTrimmedStackTraceLines(Throwable exception) {
        List<StackTraceElement> stackTraceElements = Arrays.asList(exception.getStackTrace());
        int linesToInclude = stackTraceElements.size();

        State state = State.PROCESSING_OTHER_CODE;
        for (StackTraceElement stackTraceElement : asReversedList(stackTraceElements)) {
            state = state.processStackTraceElement(stackTraceElement);
            if (state == State.DONE) {
                List<String> trimmedLines = new ArrayList<String>(linesToInclude + 2);
                trimmedLines.add("");
                for (StackTraceElement each : stackTraceElements.subList(0, linesToInclude)) {
                    trimmedLines.add("\tat " + each);
                }
                if (exception.getCause() != null) {
                    trimmedLines.add("\t... " + (stackTraceElements.size() - trimmedLines.size()) + " trimmed");
                }
                return trimmedLines;
            }
            linesToInclude--;
        }
        return Collections.emptyList();
    }

    private static List<String> getCauseStackTraceLines(Throwable exception) {
        if (exception.getCause() != null) {
            String fullTrace = getFullStackTrace(exception);
            BufferedReader reader = new BufferedReader(
                    new StringReader(fullTrace.substring(exception.toString().length())));
            List<String> causedByLines = new ArrayList<String>();
    
            try {
                String line;
                while ((line = reader.readLine()) != null) {
                    if (line.startsWith("Caused by: ")) {
                        causedByLines.add(line);
                        while ((line = reader.readLine()) != null) {
                            causedByLines.add(line);
                        }
                        return causedByLines;
                    }
                }
            } catch (IOException e) {
                // We should never get here, because we are reading from a StringReader
            }
        }

        return Collections.emptyList();
    }

    private static String getFullStackTrace(Throwable exception) {
        StringWriter stringWriter = new StringWriter();
        PrintWriter writer = new PrintWriter(stringWriter);
        exception.printStackTrace(writer);
        return stringWriter.toString();
    }

    private static void appendStackTraceLines(
            List<String> stackTraceLines, StringBuilder destBuilder) {
        for (String stackTraceLine : stackTraceLines) {
            destBuilder.append(String.format("%s%n", stackTraceLine));
        }
    }

    private static <T> List<T> asReversedList(final List<T> list) {
        return new AbstractList<T>() {

            @Override
            public T get(int index) {
                return list.get(list.size() - index - 1);
            }

            @Override
            public int size() {
                return list.size();
            }
        };
    }

    private enum State {
        PROCESSING_OTHER_CODE {
            @Override public State processLine(String methodName) {
                if (isTestFrameworkMethod(methodName)) {
                    return PROCESSING_TEST_FRAMEWORK_CODE;
                }
                return this;
            }
        },
        PROCESSING_TEST_FRAMEWORK_CODE {
            @Override public State processLine(String methodName) {
                if (isReflectionMethod(methodName)) {
                    return PROCESSING_REFLECTION_CODE;
                } else if (isTestFrameworkMethod(methodName)) {
                    return this;
                }
                return PROCESSING_OTHER_CODE;
            } 
        },
        PROCESSING_REFLECTION_CODE {
            @Override public State processLine(String methodName) {
                if (isReflectionMethod(methodName)) {
                    return this;
                } else if (isTestFrameworkMethod(methodName)) {
                    // This is here to handle TestCase.runBare() calling TestCase.runTest().
                    return PROCESSING_TEST_FRAMEWORK_CODE;
                }
                return DONE;
            } 
        },
        DONE {
            @Override public State processLine(String methodName) {
                return this;
            } 
        };

        /** Processes a stack trace element method name, possibly moving to a new state. */
        protected abstract State processLine(String methodName);
        
        /** Processes a stack trace element, possibly moving to a new state. */
        public final State processStackTraceElement(StackTraceElement element) {
            return processLine(element.getClassName() + "." + element.getMethodName() + "()");
        }
    }

    private static final String[] TEST_FRAMEWORK_METHOD_NAME_PREFIXES = {
        "org.junit.runner.",
        "org.junit.runners.",
        "org.junit.experimental.runners.",
        "org.junit.internal.",
        "junit.",
    };

    private static final String[] TEST_FRAMEWORK_TEST_METHOD_NAME_PREFIXES = {
        "org.junit.internal.StackTracesTest",
    };

    private static boolean isTestFrameworkMethod(String methodName) {
        return isMatchingMethod(methodName, TEST_FRAMEWORK_METHOD_NAME_PREFIXES) &&
                !isMatchingMethod(methodName, TEST_FRAMEWORK_TEST_METHOD_NAME_PREFIXES);
    }
    
    private static final String[] REFLECTION_METHOD_NAME_PREFIXES = {
        "sun.reflect.",
        "java.lang.reflect.",
        "org.junit.rules.RunRules.<init>(",
        "org.junit.rules.RunRules.applyAll(", // calls TestRules
        "org.junit.runners.BlockJUnit4ClassRunner.withMethodRules(", // calls MethodRules
        "junit.framework.TestCase.runBare(", // runBare() directly calls setUp() and tearDown()
   };
    
    private static boolean isReflectionMethod(String methodName) {
        return isMatchingMethod(methodName, REFLECTION_METHOD_NAME_PREFIXES);
    }

    private static boolean isMatchingMethod(String methodName, String[] methodNamePrefixes) {
        for (String methodNamePrefix : methodNamePrefixes) {
            if (methodName.startsWith(methodNamePrefix)) {
                return true;
            }
        }
        
        return false;
    }
}