限制异常栈打印行数

我们经常通过 try catch 代码块包住一段可能出现异常的代码，同时在 catch 块中打印异常信息，如下所示：

public static void main(String args[]) {
    try {
        a();
    } catch (HighLevelException e) {
        e.printStackTrace();
    }
}

当然通常情况我们会使用日志框架，不会直接使用 e.printStackTrace() 方法打印异常栈，但是日志框架同样会输出整个异常栈的信息，如下所示：

logger.error("error message", e);

大部分情况下，我们是不需要查看整个异常栈的信息就可以定位问题的，而且大量的异常栈信息堆积到日志文件中，对硬盘的消耗较大。如果同步到 ELK 中，对日志的内容进行分析，对 ES 存储的消耗也会很大。

那怎么限制异常栈的行数呢？

首先，我们来分析 e.printStackTrace() 方法的源码，看看它是如何打印异常栈信息的。

printStackTrace() 方法在 java.lang.Throwable 类中，方法的调用链路如下所示：

java.lang.Throwable#printStackTrace() -> java.lang.Throwable#printStackTrace(java.io.PrintStream) -> java.lang.Throwable#printStackTrace(java.lang.Throwable.PrintStreamOrWriter)

我们来看真正执行打印的方法：

private void printStackTrace(PrintStreamOrWriter s) {
    // Guard against malicious overrides of Throwable.equals by
    // using a Set with identity equality semantics.
    Set<Throwable> dejaVu =
        Collections.newSetFromMap(new IdentityHashMap<Throwable, Boolean>());
    dejaVu.add(this);
    synchronized (s.lock()) {
        // Print our stack trace
        s.println(this);
        StackTraceElement[] trace = getOurStackTrace();
        for (StackTraceElement traceElement : trace)
            s.println("\tat " + traceElement);
        // Print suppressed exceptions, if any
        for (Throwable se : getSuppressed())
            se.printEnclosedStackTrace(s, trace, SUPPRESSED_CAPTION, "\t", dejaVu);
        // Print cause, if any
        Throwable ourCause = getCause();
        if (ourCause != null)
            ourCause.printEnclosedStackTrace(s, trace, CAUSE_CAPTION, "", dejaVu);
    }
}
public StackTraceElement[] getStackTrace() {
    return getOurStackTrace().clone();
}
private synchronized StackTraceElement[] getOurStackTrace() {
    // Initialize stack trace field with information from
    // backtrace if this is the first call to this method
    if (stackTrace == UNASSIGNED_STACK ||
        (stackTrace == null && backtrace != null) /* Out of protocol state */) {
        int depth = getStackTraceDepth();
        stackTrace = new StackTraceElement[depth];
        for (int i=0; i < depth; i++)
            stackTrace[i] = getStackTraceElement(i);
    } else if (stackTrace == null) {
        return UNASSIGNED_STACK;
    }
    return stackTrace;
}

通过上面的代码，可以知道 e.printStackTrace() 方法中打印的内容是 StackTraceElement[]，而且在接下来会输出 suppressed exceptions 和 cause 信息。

getStackTrace() 已经提供了 public 方法，可以通过 e.getStackTrace() 方式直接获取，不需要通过反射调用下面的私有方法。

新建自定义日志帮助类，限制异常栈的打印行数。

在自定义类之前，先准备两个会输出异常信息的测试类，一个（ExceptionTest1）会输出 Caused by，一个（ExceptionTest2）会输出 Suppressed。

public class ExceptionTest1 {
    public static void main(String args[]) {
        try {
            a();
        } catch (HighLevelException e) {
            System.out.println(LoggerHelper.printTop10StackTrace(e));
        }
    }
    static void a() throws HighLevelException {
        try {
            b();
        } catch (MidLevelException e) {
            throw new HighLevelException(e);
        }
    }
    static void b() throws MidLevelException {
        c();
    }
    static void c() throws MidLevelException {
        try {
            d();
        } catch (LowLevelException e) {
            throw new MidLevelException(e);
        }
    }
    static void d() throws LowLevelException {
        e();
    }
    static void e() throws LowLevelException {
        throw new LowLevelException();
    }
}
class HighLevelException extends Exception {
    HighLevelException(Throwable cause) {
        super(cause);
    }
}
class MidLevelException extends Exception {
    MidLevelException(Throwable cause) {
        super(cause);
    }
}
class LowLevelException extends Exception {
}

public class ExceptionTest2 {
    static class ResourceB implements AutoCloseable {
        public void read() throws Exception {
            throw new Exception("ResourceB read exception");
        }
        @Override
        public void close() throws Exception {
            throw new Exception("ResourceB close exception");
        }
    }
    static class ResourceA implements AutoCloseable {
        public void read() throws Exception {
            throw new Exception("ResourceA read exception");
        }
        @Override
        public void close() throws Exception {
            throw new Exception("ResourceA close exception");
        }
    }
    public static void test() throws Exception {
        try (ResourceA a = new ResourceA();
             ResourceB b = new ResourceB()) {
            a.read();
            b.read();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
    public static void main(String[] args) throws Exception {
        test();
    }
}

自定义日志帮助类 - 1.0 版本：

import org.apache.commons.lang3.ArrayUtils;
public class LoggerHelper {
    private static final String SEPARATOR = "\r\n";
    public static String printTop10StackTrace(Throwable e) {
        return printStackTrace(e, 20);
    }
    public static String printStackTrace(Throwable e, int maxLineCount) {
        StringBuilder sb = new StringBuilder(maxLineCount * 5);
        sb.append(e.toString());
        sb.append(SEPARATOR);
        StackTraceElement[] trace = e.getStackTrace();
        if (trace == null) {
            return e.toString();
        }
        int count = maxLineCount > trace.length ? trace.length : maxLineCount;
        for (int i = 0; i < count; i++) {
            sb.append("\tat ").append(trace[i]).append(SEPARATOR);
        }
        // Print suppressed exceptions, if any
        Throwable[] suppressedExceptions = e.getSuppressed();
        if (ArrayUtils.isNotEmpty(suppressedExceptions)) {
            sb.append("\tSuppressed: ");
            for (Throwable suppressedException : suppressedExceptions) {
                sb.append(printStackTrace(suppressedException, maxLineCount));
            }
        }
        // Print cause, if any
        Throwable cause = e.getCause();
        if (cause != null) {
            sb.append("Caused by: ");
            sb.append(printStackTrace(cause, maxLineCount));
        }
        return sb.toString();
    }
}

在 ExceptionTest1 类中使用该帮助类，如下所示：

public static void main(String args[]) {
    try {
        a();
    } catch (HighLevelException e) {
        System.out.println(LoggerHelper.printTop10StackTrace(e));
    }
}

打印的结果如下所示：

D:\tools\Java\jdk1.8.0_231\bin\java.exe "-javaagent:D:\tools\JetBrains\IntelliJ IDEA 2018.3.6\lib\idea_rt.jar=51125:D:\tools\JetBrains\IntelliJ IDEA 2018.3.6\bin" -Dfile.encoding=UTF-8 -classpath D:\tools\Java\jdk1.8.0_231\jre\lib\charsets.jar;D:\tools\Java\jdk1.8.0_231\jre\lib\deploy.jar;D:\tools\Java\jdk1.8.0_231\jre\lib\ext\access-bridge-64.jar;D:\tools\Java\jdk1.8.0_231\jre\lib\ext\cldrdata.jar;D:\tools\Java\jdk1.8.0_231\jre\lib\ext\dnsns.jar;D:\tools\Java\jdk1.8.0_231\jre\lib\ext\jaccess.jar;D:\tools\Java\jdk1.8.0_231\jre\lib\ext\jfxrt.jar;D:\tools\Java\jdk1.8.0_231\jre\lib\ext\localedata.jar;D:\tools\Java\jdk1.8.0_231\jre\lib\ext\nashorn.jar;D:\tools\Java\jdk1.8.0_231\jre\lib\ext\sunec.jar;D:\tools\Java\jdk1.8.0_231\jre\lib\ext\sunjce_provider.jar;D:\tools\Java\jdk1.8.0_231\jre\lib\ext\sunmscapi.jar;D:\tools\Java\jdk1.8.0_231\jre\lib\ext\sunpkcs11.jar;D:\tools\Java\jdk1.8.0_231\jre\lib\ext\zipfs.jar;D:\tools\Java\jdk1.8.0_231\jre\lib\javaws.jar;D:\tools\Java\jdk1.8.0_231\jre\lib\jce.jar;D:\tools\Java\jdk1.8.0_231\jre\lib\jfr.jar;D:\tools\Java\jdk1.8.0_231\jre\lib\jfxswt.jar;D:\tools\Java\jdk1.8.0_231\jre\lib\jsse.jar;D:\tools\Java\jdk1.8.0_231\jre\lib\management-agent.jar;D:\tools\Java\jdk1.8.0_231\jre\lib\plugin.jar;D:\tools\Java\jdk1.8.0_231\jre\lib\resources.jar;D:\tools\Java\jdk1.8.0_231\jre\lib\rt.jar;D:\projects\test\target\classes;D:\tools\apache-maven-3.2.1\repo\net\javacrumbs\json-unit\json-unit-assertj\2.14.0\json-unit-assertj-2.14.0.jar;D:\tools\apache-maven-3.2.1\repo\net\javacrumbs\json-unit\json-unit-core\2.14.0\json-unit-core-2.14.0.jar;D:\tools\apache-maven-3.2.1\repo\org\opentest4j\opentest4j\1.1.1\opentest4j-1.1.1.jar;D:\tools\apache-maven-3.2.1\repo\org\hamcrest\hamcrest-core\2.1\hamcrest-core-2.1.jar;D:\tools\apache-maven-3.2.1\repo\org\hamcrest\hamcrest\2.1\hamcrest-2.1.jar;D:\tools\apache-maven-3.2.1\repo\org\assertj\assertj-core\3.15.0\assertj-core-3.15.0.jar;D:\tools\apache-maven-3.2.1\repo\net\javacrumbs\json-unit\json-unit-json-path\2.14.0\json-unit-json-path-2.14.0.jar;D:\tools\apache-maven-3.2.1\repo\com\jayway\jsonpath\json-path\2.4.0\json-path-2.4.0.jar;D:\tools\apache-maven-3.2.1\repo\net\minidev\json-smart\2.3\json-smart-2.3.jar;D:\tools\apache-maven-3.2.1\repo\net\minidev\accessors-smart\1.2\accessors-smart-1.2.jar;D:\tools\apache-maven-3.2.1\repo\org\ow2\asm\asm\5.0.4\asm-5.0.4.jar;D:\tools\apache-maven-3.2.1\repo\org\slf4j\slf4j-api\1.7.25\slf4j-api-1.7.25.jar;D:\tools\apache-maven-3.2.1\repo\com\fasterxml\jackson\core\jackson-core\2.10.3\jackson-core-2.10.3.jar;D:\tools\apache-maven-3.2.1\repo\com\fasterxml\jackson\core\jackson-annotations\2.10.3\jackson-annotations-2.10.3.jar;D:\tools\apache-maven-3.2.1\repo\com\fasterxml\jackson\core\jackson-databind\2.10.3\jackson-databind-2.10.3.jar;D:\tools\apache-maven-3.2.1\repo\com\fasterxml\jackson\dataformat\jackson-dataformat-cbor\2.10.3\jackson-dataformat-cbor-2.10.3.jar;D:\tools\apache-maven-3.2.1\repo\org\apache\commons\commons-lang3\3.4\commons-lang3-3.4.jar;D:\tools\apache-maven-3.2.1\repo\commons-collections\commons-collections\3.2.2\commons-collections-3.2.2.jar;D:\tools\apache-maven-3.2.1\repo\com\google\guava\guava\29.0-jre\guava-29.0-jre.jar;D:\tools\apache-maven-3.2.1\repo\com\google\guava\failureaccess\1.0.1\failureaccess-1.0.1.jar;D:\tools\apache-maven-3.2.1\repo\com\google\guava\listenablefuture\9999.0-empty-to-avoid-conflict-with-guava\listenablefuture-9999.0-empty-to-avoid-conflict-with-guava.jar;D:\tools\apache-maven-3.2.1\repo\com\google\code\findbugs\jsr305\3.0.2\jsr305-3.0.2.jar;D:\tools\apache-maven-3.2.1\repo\org\checkerframework\checker-qual\2.11.1\checker-qual-2.11.1.jar;D:\tools\apache-maven-3.2.1\repo\com\google\errorprone\error_prone_annotations\2.3.4\error_prone_annotations-2.3.4.jar;D:\tools\apache-maven-3.2.1\repo\com\google\j2objc\j2objc-annotations\1.3\j2objc-annotations-1.3.jar;D:\tools\apache-maven-3.2.1\repo\commons-beanutils\commons-beanutils\1.9.4\commons-beanutils-1.9.4.jar;D:\tools\apache-maven-3.2.1\repo\commons-logging\commons-logging\1.2\commons-logging-1.2.jar test4.ExceptionTest1
test4.HighLevelException: test4.MidLevelException: test4.LowLevelException
    at test4.ExceptionTest1.a(ExceptionTest1.java:16)
    at test4.ExceptionTest1.main(ExceptionTest1.java:6)
Caused by: test4.MidLevelException: test4.LowLevelException
    at test4.ExceptionTest1.c(ExceptionTest1.java:28)
    at test4.ExceptionTest1.b(ExceptionTest1.java:21)
    at test4.ExceptionTest1.a(ExceptionTest1.java:14)
    at test4.ExceptionTest1.main(ExceptionTest1.java:6)
Caused by: test4.LowLevelException
    at test4.ExceptionTest1.e(ExceptionTest1.java:37)
    at test4.ExceptionTest1.d(ExceptionTest1.java:33)
    at test4.ExceptionTest1.c(ExceptionTest1.java:26)
    at test4.ExceptionTest1.b(ExceptionTest1.java:21)
    at test4.ExceptionTest1.a(ExceptionTest1.java:14)
    at test4.ExceptionTest1.main(ExceptionTest1.java:6)
Process finished with exit code 0

对比 e.printStackTrace(); 方法异常栈的数据，稍稍有点区别：

查看源码，发现在打印 cause 数据的时候使用的是 java.lang.Throwable#printEnclosedStackTrace 方法，在该方法中对比了上层的异常栈信息，不会重复打印上层已经输出的内容。

private void printEnclosedStackTrace(PrintStreamOrWriter s,
                                     StackTraceElement[] enclosingTrace,
                                     String caption,
                                     String prefix,
                                     Set<Throwable> dejaVu) {
    assert Thread.holdsLock(s.lock());
    if (dejaVu.contains(this)) {
        s.println("\t[CIRCULAR REFERENCE:" + this + "]");
    } else {
        dejaVu.add(this);
        // Compute number of frames in common between this and enclosing trace
        StackTraceElement[] trace = getOurStackTrace();
        int m = trace.length - 1;
        int n = enclosingTrace.length - 1;
        while (m >= 0 && n >=0 && trace[m].equals(enclosingTrace[n])) {
            m--; n--;
        }
        int framesInCommon = trace.length - 1 - m;
        // Print our stack trace
        s.println(prefix + caption + this);
        for (int i = 0; i <= m; i++)
            s.println(prefix + "\tat " + trace[i]);
        if (framesInCommon != 0)
            s.println(prefix + "\t... " + framesInCommon + " more");
        // Print suppressed exceptions, if any
        for (Throwable se : getSuppressed())
            se.printEnclosedStackTrace(s, trace, SUPPRESSED_CAPTION,
                                       prefix +"\t", dejaVu);
        // Print cause, if any
        Throwable ourCause = getCause();
        if (ourCause != null)
            ourCause.printEnclosedStackTrace(s, trace, CAUSE_CAPTION, prefix, dejaVu);
    }
}

参考 printEnclosedStackTrace() 方法，实现 2.0 版本：

public class LoggerHelper {
    private static final String SEPARATOR = "\r\n";
    private static final String CAUSE_CAPTION = "Caused by: ";
    private static final String SUPPRESSED_CAPTION = "Suppressed: ";
    /**
     * 默认返回前10行异常栈信息
     *
     * @param e
     * @return
     */
    public static String printTop10StackTrace(Throwable e) {
        if (e == null) {
            return "";
        }
        return printStackTrace(e, 20);
    }
    public static String printStackTrace(Throwable e, int maxLineCount) {
        if (e == null || maxLineCount <= 0) {
            return "";
        }
        StringBuilder sb = new StringBuilder(maxLineCount * 10);
        sb.append(e.toString()).append(SEPARATOR);
        StackTraceElement[] trace = e.getStackTrace();
        if (trace == null) {
            return e.toString();
        }
        int count = maxLineCount > trace.length ? trace.length : maxLineCount;
        int framesInCommon = trace.length - count;
        for (int i = 0; i < count; i++) {
            sb.append("\tat ").append(trace[i]).append(SEPARATOR);
        }
        if (framesInCommon != 0) {
            sb.append("\t... ").append(framesInCommon).append(" more").append(SEPARATOR);
        }
        // Print suppressed exceptions, if any
        Throwable[] suppressedExceptions = e.getSuppressed();
        if (ArrayUtils.isNotEmpty(suppressedExceptions)) {
            for (Throwable suppressedException : suppressedExceptions) {
                sb.append(printEnclosedStackTrace(suppressedException, maxLineCount, trace, SUPPRESSED_CAPTION, "\t"));
            }
        }
        // Print cause, if any
        Throwable cause = e.getCause();
        if (cause != null) {
            sb.append(printEnclosedStackTrace(cause, maxLineCount, trace, CAUSE_CAPTION, ""));
        }
        return sb.toString();
    }
    private static String printEnclosedStackTrace(Throwable e, int maxLineCount, StackTraceElement[] enclosingTrace,
                                                  String caption, String prefix) {
        StringBuilder sb = new StringBuilder(maxLineCount * 5);
        StackTraceElement[] trace = e.getStackTrace();
        int m = trace.length - 1;
        int n = enclosingTrace.length - 1;
        while (m >= 0 && n >= 0 && trace[m].equals(enclosingTrace[n])) {
            m--;
            n--;
        }
        int count = maxLineCount > (m + 1) ? (m + 1) : maxLineCount;
        int framesInCommon = trace.length - count;
        // Print our stack trace
        sb.append(prefix).append(caption).append(e.toString()).append(SEPARATOR);
        for (int i = 0; i < count; i++) {
            sb.append(prefix).append("\tat ").append(trace[i]).append(SEPARATOR);
        }
        if (framesInCommon != 0) {
            sb.append(prefix).append("\t... ").append(framesInCommon).append(" more").append(SEPARATOR);
        }
        // Print suppressed exceptions, if any
        Throwable[] suppressedExceptions = e.getSuppressed();
        if (ArrayUtils.isNotEmpty(suppressedExceptions)) {
            for (Throwable suppressedException : suppressedExceptions) {
                sb.append(printEnclosedStackTrace(suppressedException, maxLineCount, trace, SUPPRESSED_CAPTION, prefix + "\t"));
            }
        }
        // Print cause, if any
        Throwable cause = e.getCause();
        if (cause != null) {
            sb.append(printEnclosedStackTrace(cause, maxLineCount, trace, CAUSE_CAPTION, prefix));
        }
        return sb.toString();
    }
}

上面的类基本实现了对所有层次的异常栈都进行了打印行数的限制，且不会重复打印上层已经输出的内容。

除了像上面那样在打印日志的时候手动指定每层异常栈的行数，也可以通过配置文件，设置一些默认异常可以打印全部异常栈信息，或者设置一些默认异常不打印异常栈信息。

作者：殷建卫 链接：https://www.yuque.com/yinjianwei/vyrvkf/okpfav 来源：殷建卫 - 架构笔记 著作权归作者所有。商业转载请联系作者获得授权，非商业转载请注明出处。